C:\mywork>java Hello.java
Exception in thread "main" java.lang.NoClassDefFoundError: Hello/java
or
C:\mywork>java Hello.class
Exception in thread "main" java.lang.NoClassDefFoundError: Hello/class
The 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: main
This 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)
non-static method printMsg(java.lang.String) cannot be
referenced from a static context
printMsg(s);
^
The method printMsg is called directly from main,
without any dot-prefix, and 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.
cannot find symbol
symbol : class EasySound
location: class Hello
EasySound console = new EasySound();
^
Unless installed as a package and properly imported, files for classes used in the program (in this case EasySound.java or EasySound.class) should be available in the same folder as Hello.java.
This error message may also show up when a library class is not imported. For example:
cannot find symbol
symbol : class Scanner
location: class Hello
Scanner kboard = new Scanner(System.in);
^
You need
import java.util.Scanner;
at the top of your program.
Another possible reason for this error message is
incorrect or misspelled primitive data type name. For example:
private bool match(String word, int row, int col, int rowStep, int colStep)
gives
cannot find symbol
symbol : class bool
location: class WordSearch
private bool match(String word, int row, int col, int rowStep, int colStep)
^
It should be boolean.
cannot find symbol
symbol : method PrintMsg(java.lang.String)
location: class Hello
PrintMsg(s);
^
This error occurs 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 parameters, 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:
cannot find symbol
symbol : 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 parameters instead of one and no println method exists that takes
two String parameters.
cannot find symbol
symbol : variable i
location: class java.lang.String
for (i = 0; i < n; i++))
^
A very common error, cannot find 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
for (int i = 0; i < n; i++)
<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:
<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:
<identifier> expected
public static void printMsg(msg)
^
')' expected
}
^
cannot find symbol
symbol : class msg
location: class Hello
public static void printMsg(msg)
^
missing method body, or declare abstract
public static void printMsg(msg)
^
4 errors
It should be:
public static void printMsg(String msg)
incompatible types
found : int
required: 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:
incompatible types
found : int
required: boolean
return row = 0 && row < myRows && col >= 0 && col < myCols;
^
operator && cannot be applied to int,boolean
return row = 0 && row < myRows && col >= 0 && col < myCols;
^
2 errors
An extraneous space between ! and = in an != operator may give several errors, including "incompatible types":
')' expected
if (s ! = null)
^
illegal start of expression
}
^
';' expected
}
^
incompatible types
found : java.lang.String
required: boolean
if (s ! = null)
^
4 errors
Another situation with "incompatible types" is when a literal string is used in place of a char constant or vice-versa. For example:
incompatible types
found : java.lang.String
required: char
grid[r][c] = "*";
^
Should be
grid[r][c] = '*';