Java compiler class error

Lesson: Common Problems (and Their Solutions)

Compiler Problems

Common Error Messages on Microsoft Windows Systems

‘javac’ is not recognized as an internal or external command, operable program or batch file

If you receive this error, Windows cannot find the compiler ( javac ).

Here’s one way to tell Windows where to find javac . Suppose you installed the JDK in C:\jdk1.8.0 . At the prompt you would type the following command and press Enter:

If you choose this option, you’ll have to precede your javac and java commands with C:\jdk1.8.0\bin\ each time you compile or run a program. To avoid this extra typing, consult the section Updating the PATH variable in the JDK 8 installation instructions.

Class names, ‘HelloWorldApp’, are only accepted if annotation processing is explicitly requested

If you receive this error, you forgot to include the .java suffix when compiling the program. Remember, the command is javac HelloWorldApp.java not javac HelloWorldApp .

Common Error Messages on UNIX Systems

javac: Command not found

If you receive this error, UNIX cannot find the compiler, javac .

Here’s one way to tell UNIX where to find javac . Suppose you installed the JDK in /usr/local/jdk1.8.0 . At the prompt you would type the following command and press Return:

Note: If you choose this option, each time you compile or run a program, you’ll have to precede your javac and java commands with /usr/local/jdk1.8.0/ . To avoid this extra typing, you could add this information to your PATH variable. The steps for doing so will vary depending on which shell you are currently running.

Class names, ‘HelloWorldApp’, are only accepted if annotation processing is explicitly requested

If you receive this error, you forgot to include the .java suffix when compiling the program. Remember, the command is javac HelloWorldApp.java not javac HelloWorldApp .

Syntax Errors (All Platforms)

If you mistype part of a program, the compiler may issue a syntax error. The message usually displays the type of the error, the line number where the error was detected, the code on that line, and the position of the error within the code. Here’s an error caused by omitting a semicolon ( ; ) at the end of a statement:

If you see any compiler errors, then your program did not successfully compile, and the compiler did not create a .class file. Carefully verify the program, fix any errors that you detect, and try again.

Semantic Errors

In addition to verifying that your program is syntactically correct, the compiler checks for other basic correctness. For example, the compiler warns you each time you use a variable that has not been initialized:

Again, your program did not successfully compile, and the compiler did not create a .class file. Fix the error and try again.

Runtime Problems

Error Messages on Microsoft Windows Systems

Exception in thread «main» java.lang.NoClassDefFoundError: HelloWorldApp

If you receive this error, java cannot find your bytecode file, HelloWorldApp.class .

One of the places java tries to find your .class file is your current directory. So if your .class file is in C:\java , you should change your current directory to that. To change your directory, type the following command at the prompt and press Enter:

The prompt should change to C:\java> . If you enter dir at the prompt, you should see your .java and .class files. Now enter java HelloWorldApp again.

If you still have problems, you might have to change your CLASSPATH variable. To see if this is necessary, try clobbering the classpath with the following command.

Now enter java HelloWorldApp again. If the program works now, you’ll have to change your CLASSPATH variable. To set this variable, consult the Updating the PATH variable section in the JDK 8 installation instructions. The CLASSPATH variable is set in the same manner.

Could not find or load main class HelloWorldApp.class

A common mistake made by beginner programmers is to try and run the java launcher on the .class file that was created by the compiler. For example, you’ll get this error if you try to run your program with java HelloWorldApp.class instead of java HelloWorldApp . Remember, the argument is the name of the class that you want to use, not the filename.

Exception in thread «main» java.lang.NoSuchMethodError: main

The Java VM requires that the class you execute with it have a main method at which to begin execution of your application. A Closer Look at the «Hello World!» Application discusses the main method in detail.

Error Messages on UNIX Systems

Exception in thread «main» java.lang.NoClassDefFoundError: HelloWorldApp

If you receive this error, java cannot find your bytecode file, HelloWorldApp.class .

One of the places java tries to find your bytecode file is your current directory. So, for example, if your bytecode file is in /home/jdoe/java , you should change your current directory to that. To change your directory, type the following command at the prompt and press Return:

If you enter pwd at the prompt, you should see /home/jdoe/java . If you enter ls at the prompt, you should see your .java and .class files. Now enter java HelloWorldApp again.

If you still have problems, you might have to change your CLASSPATH environment variable. To see if this is necessary, try clobbering the classpath with the following command.

Now enter java HelloWorldApp again. If the program works now, you’ll have to change your CLASSPATH variable in the same manner as the PATH variable above.

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Exception in thread «main» java.lang.NoClassDefFoundError: HelloWorldApp/class

A common mistake made by beginner programmers is to try and run the java launcher on the .class file that was created by the compiler. For example, you’ll get this error if you try to run your program with java HelloWorldApp.class instead of java HelloWorldApp . Remember, the argument is the name of the class that you want to use, not the filename.

Exception in thread «main» java.lang.NoSuchMethodError: main

The Java VM requires that the class you execute with it have a main method at which to begin execution of your application. A Closer Look at the «Hello World!» Application discusses the main method in detail.

Applet or Java Web Start Application Is Blocked

If you are running an application through a browser and get security warnings that say the application is blocked, check the following items:

Verify that the attributes in the JAR file manifest are set correctly for the environment in which the application is running. The Permissions attribute is required. In a NetBeans project, you can open the manifest file from the Files tab of the NetBeans IDE by expanding the project folder and double-clicking manifest.mf.

Verify that the application is signed by a valid certificate and that the certificate is located in the Signer CA keystore.

If you are running a local applet, set up a web server to use for testing. You can also add your application to the exception site list, which is managed in the Security tab of the Java Control Panel.

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Java Compiler Error: “class, interface, or enum expected”

Table of Contents

Introduction

Java errors are the lifelong enemy of every developer, be it a novice or an expert. A Java developer faces a plethora of different types of errors. One such error is the class interface or enum expected error.

In this article, we will be focusing on the reason behind the occurrences of this error and how to resolve it.

Class interface or enum expected error

The class interface or enum expected error is a compile-time error in Java. It is mainly faced by the developers at their early stages in Java development.

The primary reason behind the class interface or enum expected error is the incorrect number of curly braces. Typically, this error is faced when there is an excess or shortage of a curly brace at the end of the code.

Since the whole code is placed inside a class, interface, or enum in Java, an extra curly brace makes the compiler understand that another class is starting and no closing braces after that is considered as the incompletion of class hence it will complain about class, interface, or enum keyword.

We will be now discussing some of the basic causes of class, interface, or enum expected error and how you can fix them.

1. Misplaced Curly Braces

The primary cause of the “class, interface or enum expected” error is typically a mistyped curly brace “>” in your code.

This error could have been encountered due to either an extra curly brace after the class or due to a missed curly brace in your code.

Look at this example below:

In the above code demonstration, there is an extra “>” curly brace at the last which is resulting in the compilation error. The removal of the extra “>” can easily resolve the error in this case.

2. A Function is declared outside of the class

Let’s look at another scenario where this error usually occurs:

In the above example, the class, interface, or enum expected error is faced because the function printHello() is defined outside of the class.

Although, this error is also somewhat due to misplacing of curly braces but it is important to identify this different cause so that the user would also quickly look at all the methods and their placement in the code to make sure that all functions are properly placed.

This can be easily fixed by just moving the closing curly braces “>” to the end of the class so that the printHello() method will be now inside the MyClass.

3. Class is not declared

You would also face this error if you have not declared the class. There might be a chance that you forgot to declare the class at all.

Always make sure that the class, interface, or enum is properly declared in your java file.

4. Declaration of multiple packages in the same file

More than one package cannot be present in the same Java source file. It will result in the class interface or enum expected error if your source file contains more than one package.

See the code example below where two packages are present in the same code. You must avoid this in your code:

Tips to prevent the “class, interface, or enum expected” error in Java

All the codes that we have discussed above consisted of very limited lines which makes it very easy for users to spot the misplaced curly brace but it will not be that simple if we have to look for it in a huge code with multiple methods and classes.

·Use a modern IDE

The use of an IDE can be a very good solution for preventing this error.

Various new and modern IDEs have features that automatically add the missing curly braces as detected or they right away highlights the extra added braces in your code before compilation.

Despite that, even if the error occurs, modern IDEs like Eclipse or Netbeans also give the user a visible sign of where the error is located and it will pinpoint the exact location of the error in your code.

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·Indent your code

Assuming that you are not able to use an IDE, if you are writing your code in a word processing software such as Notepad, then try to correctly indent your code.

The clear indentations make it easier to identify if there are extra curly braces at the end of the code as they would be at the same indentation level, which should not be part of a valid code.

You can simply remove the extra curly braces for the code before compiling it and the class interface or enum expected error can be easily prevented.

Wrapping it up

We discussed various reasons behind the occurrence of class, interface, or enum expected error. We also looked into the same ways to prevent this error.

It is a very trivial error and can be very quickly solved but it can sometimes become a bit troubling especially when occurred in a big code. To cater to that, always use a modern IDE to prevent this error.

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Java compiler class error

Table of Contents

When a program violates the semantic constraints of the Java programming language, the Java Virtual Machine signals this error to the program as an exception .

An example of such a violation is an attempt to index outside the bounds of an array. Some programming languages and their implementations react to such errors by peremptorily terminating the program; other programming languages allow an implementation to react in an arbitrary or unpredictable way. Neither of these approaches is compatible with the design goals of the Java SE platform: to provide portability and robustness.

Instead, the Java programming language specifies that an exception will be thrown when semantic constraints are violated and will cause a non-local transfer of control from the point where the exception occurred to a point that can be specified by the programmer.

An exception is said to be thrown from the point where it occurred and is said to be caught at the point to which control is transferred.

Programs can also throw exceptions explicitly, using throw statements (§14.18).

Explicit use of throw statements provides an alternative to the old-fashioned style of handling error conditions by returning funny values, such as the integer value -1 where a negative value would not normally be expected. Experience shows that too often such funny values are ignored or not checked for by callers, leading to programs that are not robust, exhibit undesirable behavior, or both.

Every exception is represented by an instance of the class Throwable or one of its subclasses (§11.1). Such an object can be used to carry information from the point at which an exception occurs to the handler that catches it. Handlers are established by catch clauses of try statements (§14.20).

During the process of throwing an exception, the Java Virtual Machine abruptly completes, one by one, any expressions, statements, method and constructor invocations, initializers, and field initialization expressions that have begun but not completed execution in the current thread. This process continues until a handler is found that indicates that it handles that particular exception by naming the class of the exception or a superclass of the class of the exception (§11.2). If no such handler is found, then the exception may be handled by one of a hierarchy of uncaught exception handlers (§11.3) — thus every effort is made to avoid letting an exception go unhandled.

The exception mechanism of the Java SE platform is integrated with its synchronization model (§17.1), so that monitors are unlocked as synchronized statements (§14.19) and invocations of synchronized methods (§8.4.3.6, §15.12) complete abruptly.

11.1. The Kinds and Causes of Exceptions

11.1.1. The Kinds of Exceptions

An exception is represented by an instance of the class Throwable (a direct subclass of Object ) or one of its subclasses.

Throwable and all its subclasses are, collectively, the exception classes .

The classes Exception and Error are direct subclasses of Throwable :

Exception is the superclass of all the exceptions from which ordinary programs may wish to recover.

The class RuntimeException is a direct subclass of Exception . RuntimeException is the superclass of all the exceptions which may be thrown for many reasons during expression evaluation, but from which recovery may still be possible.

RuntimeException and all its subclasses are, collectively, the run-time exception classes .

Error is the superclass of all the exceptions from which ordinary programs are not ordinarily expected to recover.

Error and all its subclasses are, collectively, the error classes .

The unchecked exception classes are the run-time exception classes and the error classes.

The checked exception classes are all exception classes other than the unchecked exception classes. That is, the checked exception classes are Throwable and all its subclasses other than RuntimeException and its subclasses and Error and its subclasses.

Programs can use the pre-existing exception classes of the Java SE platform API in throw statements, or define additional exception classes as subclasses of Throwable or of any of its subclasses, as appropriate. To take advantage of compile-time checking for exception handlers (§11.2), it is typical to define most new exception classes as checked exception classes, that is, as subclasses of Exception that are not subclasses of RuntimeException .

The class Error is a separate subclass of Throwable , distinct from Exception in the class hierarchy, to allow programs to use the idiom » > catch (Exception e) < " (§11.2.3) to catch all exceptions from which recovery may be possible without catching errors from which recovery is typically not possible.

Note that a subclass of Throwable cannot be generic (§8.1.2).

11.1.2. The Causes of Exceptions

An exception is thrown for one of three reasons:

A throw statement (§14.18) was executed.

An abnormal execution condition was synchronously detected by the Java Virtual Machine, namely:

evaluation of an expression violates the normal semantics of the Java programming language (§15.6), such as an integer divide by zero.

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an error occurs while loading, linking, or initializing part of the program (§12.2, §12.3, §12.4); in this case, an instance of a subclass of LinkageError is thrown.

an internal error or resource limitation prevents the Java Virtual Machine from implementing the semantics of the Java programming language; in this case, an instance of a subclass of VirtualMachineError is thrown.

These exceptions are not thrown at an arbitrary point in the program, but rather at a point where they are specified as a possible result of an expression evaluation or statement execution.

An asynchronous exception occurred (§11.1.3).

11.1.3. Asynchronous Exceptions

Most exceptions occur synchronously as a result of an action by the thread in which they occur, and at a point in the program that is specified to possibly result in such an exception. An asynchronous exception is, by contrast, an exception that can potentially occur at any point in the execution of a program.

Asynchronous exceptions occur only as a result of:

An invocation of the (deprecated) stop method of class Thread or ThreadGroup .

The (deprecated) stop methods may be invoked by one thread to affect another thread or all the threads in a specified thread group. They are asynchronous because they may occur at any point in the execution of the other thread or threads.

An internal error or resource limitation in the Java Virtual Machine that prevents it from implementing the semantics of the Java programming language. In this case, the asynchronous exception that is thrown is an instance of a subclass of VirtualMachineError .

Note that StackOverflowError , a subclass of VirtualMachineError , may be thrown synchronously by method invocation (§15.12.4.5) as well as asynchronously due to native method execution or Java Virtual Machine resource limitations. Similarly, OutOfMemoryError , another subclass of VirtualMachineError , may be thrown synchronously during class instance creation (§15.9.4, §12.5), array creation (§15.10.2, §10.6), class initialization (§12.4.2), and boxing conversion (§5.1.7), as well as asynchronously.

The Java SE platform permits a small but bounded amount of execution to occur before an asynchronous exception is thrown.

Asynchronous exceptions are rare, but proper understanding of their semantics is necessary if high-quality machine code is to be generated.

The delay noted above is permitted to allow optimized code to detect and throw these exceptions at points where it is practical to handle them while obeying the semantics of the Java programming language. A simple implementation might poll for asynchronous exceptions at the point of each control transfer instruction. Since a program has a finite size, this provides a bound on the total delay in detecting an asynchronous exception. Since no asynchronous exception will occur between control transfers, the code generator has some flexibility to reorder computation between control transfers for greater performance. The paper Polling Efficiently on Stock Hardware by Marc Feeley, Proc. 1993 Conference on Functional Programming and Computer Architecture, Copenhagen, Denmark, pp. 179-187, is recommended as further reading.

11.2. Compile-Time Checking of Exceptions

The Java programming language requires that a program contains handlers for checked exceptions which can result from execution of a method or constructor (§8.4.6, §8.8.5). This compile-time checking for the presence of exception handlers is designed to reduce the number of exceptions which are not properly handled. For each checked exception which is a possible result, the throws clause for the method or constructor must mention the class of that exception or one of the superclasses of the class of that exception (§11.2.3).

The checked exception classes (§11.1.1) named in the throws clause are part of the contract between the implementor and user of the method or constructor. The throws clause of an overriding method may not specify that this method will result in throwing any checked exception which the overridden method is not permitted, by its throws clause, to throw (§8.4.8.3). When interfaces are involved, more than one method declaration may be overridden by a single overriding declaration. In this case, the overriding declaration must have a throws clause that is compatible with all the overridden declarations (§9.4.1).

The unchecked exception classes (§11.1.1) are exempted from compile-time checking.

Error classes are exempted because they can occur at many points in the program and recovery from them is difficult or impossible. A program declaring such exceptions would be cluttered, pointlessly. Sophisticated programs may yet wish to catch and attempt to recover from some of these conditions.

Run-time exception classes are exempted because, in the judgment of the designers of the Java programming language, having to declare such exceptions would not aid significantly in establishing the correctness of programs. Many of the operations and constructs of the Java programming language can result in exceptions at run time. The information available to a Java compiler, and the level of analysis a compiler performs, are usually not sufficient to establish that such run-time exceptions cannot occur, even though this may be obvious to the programmer. Requiring such exception classes to be declared would simply be an irritation to programmers.

For example, certain code might implement a circular data structure that, by construction, can never involve null references; the programmer can then be certain that a NullPointerException cannot occur, but it would be difficult for a Java compiler to prove it. The theorem-proving technology that is needed to establish such global properties of data structures is beyond the scope of this specification.

We say that a statement or expression can throw an exception class E if, according to the rules in §11.2.1 and §11.2.2, the execution of the statement or expression can result in an exception of class E being thrown.

We say that a catch clause can catch its catchable exception class(es):

The catchable exception class of a uni- catch clause is the declared type of its exception parameter (§14.20).

The catchable exception classes of a multi- catch clause are the alternatives in the union that denotes the type of its exception parameter.

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