Java error messages

Error messages in the console when compiling and running code are not always self-explanatory. To give you a better overview of what the errors mean and how to fix them, the most common errors are listed here.

Depending on the Java version you are using, messages may appear in English or German.

What should I do if I still cannot figure it out?

Look up the error message:
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Compile errors

Array required, but <X> found

array required but String found
    String b = a[0];
            ^

Possible causes

• Accessing an index with [] on a variable that is not an array type

Possible solutions

• Check the type of the variable
• Use the appropriate access method for the type, e.g. "get()" for lists

ERROR_array_required

Cannot find symbol

cannot find symbol
    positionX = x;
    ^
symbol: variable positionX
location: class Block

Possible causes

• Variable was not defined
• Name is misspelled
• Missing "new" when creating an object

Possible solutions

• Make sure the variable being used has been defined
• the first occurrence of a variable must include a data type
• Check for spelling mistakes in the name

Note:

The search feature can help you find a variable and its usage more quickly.

ERROR_cannot_find_symbol

class, interface, enum, or record expected

class, interface, enum, or record expected
  }
  ^

Possible causes

• Too many closing curly braces (})

Possible solutions

• Remove the extra braces

Note:

The extra braces are usually found at the end of the code for this error. Proper code indentation can help you find and avoid this error.

ERROR_class_expected

class, interface, annotation type, enum, record, method or field expected

class, interface, annotation type, enum, record, method or field expected
publi void begruessung(String name) {
^

Possible causes

• Invalid modifier (static, public, ...)

Possible solutions

• Fix or remove the modifier

ERROR_class_interface_expected

Invalid method declaration; return type required

invalid method declaration; return type required
begruessung(String name) {
^

Possible causes

• Missing return type (void, int, boolean...)

Possible solutions

• Add the return type

ERROR_invalid_method_declaration

abc

Class <X> is public, should be declared in a file named <X>.java

class Block is public, should be declared in a file named Block.java
public class Block {
       ^

Possible causes

• The class name does not match the file name

Possible solutions

• Make the names match

Note:

When renaming a class, the corresponding file must also be renamed. 5Code automatically renames the class when you rename the file.

ERROR_class_file_name

Duplicate case label

duplicate case label
    case 's': ausgabe = "Daten werden gespeichert";
    ^

Possible causes

• A case in a switch statement is used more than once.

Possible solutions

• Remove the duplicate case or change the literal to an unused value

Note:

When copying and pasting code from another location, make sure to adjust the code at all relevant places. (Ideally, avoid copy & paste!)

ERROR_duplicate_case_label

"else" without "if"

'else' without 'if'
    else {
    ^

Possible causes

• An else branch was found that cannot be assigned to any if statement ("dangling else").

Possible solutions

• Check your braces: wrap multiple statements under if in a block ({})
• Remove empty statements (;) after if

ERROR_else_without_if

Illegal start of expression/type

illegal start of expression
    void dreheAufWinkel(int winkel) {
    ^
illegal start of type
    return str;
    ^

Possible causes

• Missing braces
• A method is placed inside another method
• A statement is placed outside a method
• Access modifiers (public, protected, private) used inside a method

Possible solutions

• Check your braces

Note:

Proper code indentation can help you find and avoid this error. The cause of the error is very likely found in the lines before the error.

ERROR_illegal_start

Incompatible types

incompatible types: String cannot be converted to int
    int a = "a";
            ^
incompatible types: possible lossy conversion from double to int
    int x = Math.pow(2,4);
                    ^

Possible causes

• A variable is assigned a value of the wrong type

Possible solutions

• Check whether the variable was confused with another one
• Convert the value (if possible)

Note:

Floating-point numbers can also be converted to integers, e.g. int pi = (int)3.14159. Note that the values will be rounded. Strings consisting of numbers can also be converted, e.g. Integer.parseInt("500").

ERROR_incompatible_types

Invalid method declaration; return type required

invalid method declaration; return type required
    public toString() {
           ^

Possible causes

• Missing return type in the method declaration

Possible solutions

• Add the appropriate return type (void, int, String, ...)

Note:

Even if the method does not return a value, it must be explicitly declared as "void".

ERROR_return_type

Method <X> is already defined in class <Y>

method toString() is already defined in class Block
    public String toString() {
                  ^

Possible causes

• A method with this name and the given parameters has already been declared in the class

Possible solutions

• Change the method name
• Change the types or number of parameters

ERROR_method_already_defined

Method <X> in class <Y> cannot be applied to given types

method positioniereAuf in class Block cannot be applied to given types;
    block.positioniereAuf(2);
         ^
required: int,int
found:    int
reason: actual and formal argument lists differ in length

Possible causes

• The parameters of the called method do not match its definition

Possible solutions

• Adjust the parameters to match the definition (number and type)

Note:

The error message details which parameters are expected and which were provided. The search feature can help you find the method definition more quickly.

ERROR_cannot_be_applied_to_types

Missing return statement

missing return statement
    }
    ^

Possible causes

• A method with a return type (not "void") does not return a value
• "return" is not called in every case (e.g. inside an if branch)

Possible solutions

• Add "return" with the appropriate return value at the end of the method
• Check that all branches call "return"

Note:

If different values are returned under certain conditions (if), the other conditions must be introduced with "else if". Otherwise the compiler cannot guarantee that a value is returned in every case.

ERROR_missing_return

Non-static variable <X> cannot be referenced from a static context

non-static variable name cannot be referenced from a static context
    name = "Alice";
    ^

Possible causes

• An instance property of the class is being accessed from within a static method.

Possible solutions

• Define the variable as static
• Create an instance of the class and access it through the instance

ERROR_non_static_variable

Not a statement

not a statement
    abc
    ^

Possible causes

• There is no valid statement present

Possible solutions

• For method calls: add ()
• For variables: add an assignment (e.g. = otherVariable)
• Make sure comments are properly marked as comments

ERROR_not_a_statement

Package <X> does not exist

package aufgaben does not exist
    import aufgaben.*;
    ^
package system does not exist
    system.out.println("Hello world");
          ^

Possible causes

• Package name or path was specified incorrectly
• A class name was written in lowercase, e.g. when calling "System.out.println()"

Possible solutions

• Check the name and location of the imported package
• Fix lowercase class names

ERROR_package_does_not_exist

Reached end of file while parsing

reached end of file while parsing
    }
     ^

Possible causes

• Not all braces are properly closed

Possible solutions

• Add the missing braces

Note:

Proper code indentation can help you find and avoid this error.

ERROR_end_of_file

Unclosed character literal

unclosed character literal char
    zeichen = 'a;
              ^

Possible causes

• A char literal is not properly closed
• A char literal contains more than one character

Possible solutions

• Add the closing ' to the char literal
• For multiple characters, use the String data type

ERROR_unclosed_character_literal

Unreachable statement

unreachable statement
    System.out.println(str);
    ^

Possible causes

• Code lines after a "return" or "break" (within the same block)

Possible solutions

• Move the code lines before "return" or "break"

ERROR_unreachable_statement

uses unchecked or unsafe operations

Note: [...]\Super.java uses unchecked or unsafe operations.

What causes this warning?

• Using raw types with generics

How can I fix this warning?

• Assign a type parameter when using generics

ERROR_unsafe_operations

Variable <X> might not have been initialized

variable str might not have been initialized
    System.out.println(str);
                       ^

Possible causes

• Variable was not initialized
• Variable is only initialized under certain conditions

Possible solutions

• Initialize the variable before using it
• Check that all branches initialize the variable

Note:

If variables are initialized under certain conditions (if), the other conditions must be introduced with "else if". Otherwise the compiler cannot guarantee that the variable is initialized in every case.

ERROR_var_not_initialized

Variable <X> is already defined in method <Y>

variable a is already defined in method main(String[])
    int a = 9;
       ^

Possible causes

• A variable with this name has already been declared in the block

Possible solutions

• Change the variable name
• Check the scopes of the blocks

Note:

Within a block, duplicate variable names are not allowed, even if they have different types.

ERROR_var_already_defined

<identifier> expected

<identifier> expected
    System.out.println("Hello world");
                      ^

Possible causes

• Incorrect braces
• Statements are placed outside a method

Possible solutions

• Check your braces
• Move statements into a method (e.g. main())

Note:

Proper code indentation can help you find and avoid this error.

ERROR_identifier_expected

<X> expected

';' expected
    position X = 0
                  ^

Possible causes

• Missing character (e.g. ";" or "{")
• Incorrect braces

Possible solutions

• Add the missing character at the indicated position
• Check your braces

ERROR_expected

<X> has private access in <Y>

name has private access in Person
    String name = person.name;
                        ^

Possible causes

• Accessing instance variables or methods that have private visibility

Possible solutions

• For instance methods: change the visibility as needed (public, protected, default)
• For instance variables: these should remain private, and direct access to instance variables from outside must be avoided. Try to adjust your logic so that direct access is not needed.

ERROR_private_access

<X> in <Y> has been deprecated

[deprecation] stop() in Thread has been deprecated
    t.stop();
     ^
[removal] Integer(int) in Integer has been deprecated and marked for removal
    Integer i = new Integer(5);
               ^

What causes this warning?

• Methods or constructors have been used that are no longer intended for use. These are marked as "deprecated" in the API documentation. Over the course of Java's development, some methods or constructors have been revised or replaced with better, safer versions or alternative methods.

How can I fix this warning?

• Check the API documentation for the better, safer version you should use instead. There may also be hints in the assignment instructions.

ERROR_deprecation

Bad operand type <X> for unary operator "<Y>"

bad operand type boolean for unary operator '++'
        bool++;
            ^

Possible causes

• An operator is being applied to an incompatible data type.

Possible solutions

• Check whether the operator is being applied to the correct variable.
• Check whether the variable has a compatible type.
• +, -, /, *, % can be applied to two numeric types, e.g. 3 + 5
• ! can be applied to boolean, e.g. !isTooFast
• ++ and -- can be applied to numeric types, e.g. i--
• &, &&, |, || can be applied to boolean, e.g. isBig && isFast

ERROR_bad_operand

Incomparable types: <X> and <Y>

incomparable types: boolean and int
        boolean bool3 = bool == 8;
                             ^


Possible causes

• An attempt is made to compare values of two types that cannot be compared, e.g. boolean and int.
• Booleans can be compared with each other, and numeric values can be compared with each other. Mixing them is not possible.

Possible solutions

• Check whether the correct values or variables are being compared.
• Check whether the elements being compared have the correct type.

ERROR_incomparable_types

Unnamed classes are a preview feature and are disabled by default.

unnamed classes are a preview feature and are disabled by default.
    void hello() {
    ^


Possible causes

• A definition is found outside the class where it does not belong.

Possible solutions

• Check whether the block above was closed only once (}).
• Check whether a method was closed incorrectly.

ERROR_unnamed_classes

Duplicate class: X

duplicate class: Test
class Test {
^


Possible causes

• A class was named incorrectly.

Possible solutions

• Check whether the class name and file name match (ClassName.java), and adjust one of them accordingly.
• Check whether the class name is used more than once in the project. This can happen if files are copied into the project from outside of 5Code.

ERROR_duplicate_class

Array dimension missing

array dimension missing
        int[] numbers = new int[];
                                 ^


Possible causes

• The size is missing when creating an array.

Possible solutions

• Add the array length inside the brackets, e.g. new int[5].

ERROR_array_dimension_missing

'.class' expected

'.class' expected
        this.zahl = int _zahl;
                             ^


Possible causes

• A data type is used incorrectly, e.g. when assigning a value.

Possible solutions

• Make sure data types are used when defining a variable, not when assigning a value (i.e. on the left side of =).
• Make sure data types are used when defining formal parameters, not in actual arguments.

ERROR_dot_class_expected

Compact source file does not have main method in the form of void main() or void main(String[] args)

void mains() {
^


Possible causes

• A main() method is missing, which is required to run the program.

Possible solutions

• Add a "void main()" if it is missing.
• Check for typos in the method name.
• This may not be the Java file that should be run. Try running a different file that has a "void main()".

ERROR_compact_source_file_main

Runtime errors

ArrayIndexOutOfBoundsException: <X>

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: Index 4 out of bounds for length 2

Possible causes

• Attempted access to a non-existent index in an array

Possible solutions

• Check the length of the array
• Check the index calculation
• Check the loop condition that accesses the array

Note:

Array indices go from 0 to array length - 1.

ERROR_array_oob

ClassCastException: <X>

Exception in thread "main" java.lang.ClassCastException: class java.lang.Integer cannot be cast to class java.lang.String (java.lang.Integer and java.lang.String are in module java.base of loader 'bootstrap')

Possible causes

• Casting an object of one class to an object of another class fails because the types are incompatible

Possible solutions

• Check the return type of the methods being used (do they return what is expected)
• Look up which types are compatible with each other
• Check the inheritance structure of the relevant classes
• Avoid untyped collections (raw types)

ERROR_class_cast

NullPointerException

Exception in thread "main" java.lang.NullPointerException

Possible causes

• Attempted access to an element without a value (null), e.g.
  • Accessing an array element whose value is null
  • Querying the length of an array whose value is null
  • Calling methods on a variable with the value null

Possible solutions

• Make sure the referenced values or variables are set
• Check if the variable is not null before performing operations

ERROR_null_pointer

StringIndexOutOfBoundsException: <X>

Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: -1
Exception in thread "main" java.lang.StringIndexOutOfBoundsException: begin 0, end 4, length 1

Possible causes

• Attempted access to a non-existent index in a string

Possible solutions

• Check the length of the string
• Check the index calculation
• Check the loop condition that accesses the string

Note:

String indices go from 0 to string length - 1.

ERROR_string_oob

by zero

Exception in thread "main" java.lang.ExceptionInInitializerError Caused by: java.lang.ArithmeticException: / by zero

Possible causes

• Mathematical operation: division by 0

Possible solutions

• Check the values of the variables involved in the division
• Wrap the calculation in a try/catch block

ERROR_divide_by_zero