Java Methods
Common Syntax Error Messages
Exception in thread "main" java.lang.NoClassDefFoundError -- wrong name
Exception in thread "main" java.lang.NoClassDefFoundError -- /java
Exception in thread "main" java.lang.NoClassDefFoundError -- /class
Exception in thread "main" java.lang.NoSuchMethodError: main
class is public, should be declared in a file named
cannot return a value from method whose result type is void
non-static method cannot be referenced from a static context
cannot resolve symbol -- class
cannot resolve symbol -- method
cannot resolve symbol -- variable
'}' expected
'class' or 'interface' expected
illegal character
<identifier> expected
'(' or '[' expected
variable might not have been initialized
unclosed string literal
missing return statement
';' expected
incompatible types
'[' expected
array required, but java.lang.String found
possible loss of precision
'.class' expected
attempting to assign weaker access privileges C:\mywork>java hello Exception in thread "main" java.lang.NoClassDefFoundError: hello (wrong name: Hello)This run-time error (exception) may happen when you mistype a lower case letter for upper case. Normally a class name (e.g., Hello) starts with an upper case letter and the file name should be the same. SDK 1.3 under Windows will compile a file hello.java that defines a class Hello, but when you try to run it as above, it reports an exception. C:\mywork>java Hellois correct. C:\mywork>java Hello.java Exception in thread "main" java.lang.NoClassDefFoundError: Hello/javaor C:\mywork>java Hello.class Exception in thread "main" java.lang.NoClassDefFoundError: Hello/classThe command to run the Java interpreter should use the class name but should not include any extension, neither .java nor .class. An extension in the file name confuses the interpreter about the location of the class (the extension is interpreted as a subfolder).C:\mywork>java Hello Exception in thread "main" java.lang.NoSuchMethodError: mainThis exception may be reported when the main method is missing or its signature is incorrect. The correct signature is public static void main (String[] args)Possible mistakes: private static void main (String[] args)public void main (String[] args)public static int main (String[] args)public static void main (String args)C:\mywork>javac Test.java Test.java:1: class Hello is public, should be declared in a file named Hello.javapublic class Hello ^The source file name is different from the name of the class defined in the file. Here the file name is Test and the class name is Hello. They must be the same.Hello.java:8: cannot return a value from method whose result type is void return 0; ^In Java, main is void, not int, so return is not needed and you can't use return 0 in it.Hello.java:12: non-static method printMsg(java.lang.String) cannot be referenced from a static context printMsg(s); ^The keyword static is missing in the printMsg header: public void printMsg(String msg)should be: public static void printMsg(String msg)Since main is a static method and it calls printMsg with no "something-dot" prefix, printMsg is assumed to be another static method of the same class. Hello.java:7: cannot resolve symbolsymbol : class EasyReaderlocation: class Hello EasyReader console = new EasyReader(); ^Unless installed as a package and properly imported, files for classes used in the program (in this case EasyReader.java or EasyReader.class) should be available in the same folder as Hello.java. A cannot resolve class error may also show up when a library class is not imported or when data type is incorrect or misspelled. For example: private bool match(String word, int row, int col, int rowStep, int colStep)gives WordSearch.java:32: cannot resolve symbolsymbol : class boollocation: class WordSearch private bool match(String word, int row, int col, int rowStep, int colStep) ^It should be boolean. Hello.java:7: cannot resolve symbolsymbol : method PrintMsg (java.lang.String)location: class Hello PrintMsg(s); ^This error may occur when a method is called incorrectly: either its name is misspelled (or upper-lower case is misplaced), or a method is called with wrong types of arguments, or a method is called for a wrong type of object or a wrong class. For example, the same error, will be reported if you write System.println("Hello");instead of System.out.println("Hello");Another example: Hello.java:20: cannot resolve symbolsymbol : method println (java.lang.String,java.lang.String)location: class java.io.PrintStream System.out.println("You entered: ", msg); ^Here a comma is used instead of a + in the println call. This makes it a call with two arguments instead of one and no println method exists that takes two String arguments. Hello.java:18: cannot resolve symbolsymbol : variable lengthlocation: class java.lang.String if (msg.length != 0) ^A very common error, cannot resolve symbol may result from an undeclared variable or a misspelled local variable or field name, or missing parentheses in a method call. Here it should be msg.length().Hello.java:8: '}' expected } ^An extra opening brace or a missing closing brace may produce several errors, including Hello.java:13: illegal start of expression public static void printMsg(String msg) ^Hello.java:16: ';' expected } ^and finally Hello.java:17: '}' expected} ^which may be reported at the end of the file. Hello.java:29: 'class' or 'interface' expected}^This error often results from an extra closing brace (or a missing opening brace).Hello.java:5: illegal character: 20 System.out.println(鬑ello World?; ^"Smart quote" characters accidentally left in the source file by a word processor instead of straight single or double quotes may cause this error. The same error is reported when the source file contains any non-ASCII character in the code (outside comments).Hello.java:3: <identifier> expected static x; ^<identifier> expected is a rather common error message. Here x is a variable, but the compiler thinks it is a class name. It is the data type designation that's actually missing. It should be: static <someType> x;The same happens here: private myRows, myCols;It gives an error: WordSearch.java:4: <identifier> expected private myRows, myCols; ^thinking that myRows is a data type. Same here: public static void printMsg(msg) { ... }- a missing type designator (e.g., String) in a method's header produces four rather obscure errors: Hello.java:19: <identifier> expected public static void printMsg(msg) ^Hello.java:29: ')' expected } ^Hello.java:19: cannot resolve symbolsymbol : class msglocation: class Hello public static void printMsg(msg) ^Hello.java:19: missing method body, or declare abstract public static void printMsg(msg) ^4 errorsIt should be: public static void printMsg(String msg)Hello.java:7: '(' or '[' expected EasyReader inp = new EasyReader; ^Should be: EasyReader inp = new EasyReader();Hello.java:9: variable inp might not have been initialized s = inp.readLine(); ^This error happens if you use a local variable before initializing it. EasyReader inp;declares a variable but you need to initialize it with new. Hello.java:8: ')' expected System.out.print(Enter a message: "); ^Hello.java:8: unclosed string literal System.out.print(Enter a message: "); ^Hello.java:8: cannot resolve symbolsymbol : variable Enterlocation: class Hello System.out.print(Enter a message: "); ^3 errorsA missing opening double quote in a literal string produces these three errors.Hello.java:17: missing return statement { ^A method, other than void, must return a value.Hello.java:23: ';' expected System.out.println("Message: " + msg) ^A few compiler error messages are actually self-explanatory.WordSearch.java:18: incompatible typesfound : intrequired: boolean if (i = n) ^It is supposed to be if (i == n)Single = makes it assignment operator. It returns an int value that can't be tested in if. Similarly, return row = 0 && row < myRows && col >= 0 && col < myCols;gives two errors: WordSearch.java:29: incompatible typesfound : intrequired: boolean return row = 0 && row < myRows && col >= 0 && col < myCols; ^WordSearch.java:29: operator && cannot be applied to int,boolean return row = 0 && row < myRows && col >= 0 && col < myCols; ^2 errorsAn extraneous space between ! and = in an != operator may give several errors, including "incompatible types": Hello.java:10: ')' expected if (s ! = null) ^Hello.java:14: illegal start of expression } ^Hello.java:13: ';' expected } ^Hello.java:10: incompatible typesfound : java.lang.Stringrequired: boolean if (s ! = null) ^4 errorsAnother situation with "incompatible types" is when a literal string is used in place of a char constant or vice-versa. For example: WordSearch.java:21: incompatible typesfound : java.lang.Stringrequired: char grid[r][c] = "*"; ^Should be grid[r][c] = '*';WordSearch.java:10: '[' expected grid = new char(rows, cols); ^An array should be created using brackets, not parentheses.WordSearch.java:20: array required, but java.lang.String found grid[r][c] = Character.toUpperCase(letters[i]); ^Use charAt(i) method, not [i] with strings.Hello.java:7: possible loss of precisionfound : doublerequired: int x = 3.5; ^This happens when a double value is assigned to an int variable.Hello.java:8: '.class' expected double y = double(x); ^Hello.java:8: unexpected typerequired: valuefound : class double y = double(x); ^2 errorsIncorrect cast syntax causes this error. Should be double y = (double)x;Test.java:5: actionPerformed(java.awt.event.ActionEvent) in Testcannot implement actionPerformed(java.awt.event.ActionEvent) injava.awt.event.ActionListener;attempting to assign weaker access privileges; was publicpublic class Test extends JApplet ^1 errorThis error is reported when the keyword public is missing in the actionPerformed method's header.
