I know it depends on the context in which the anonymous class has been written (static or non static method).
but look this part of code:
public class A {
int fieldOfA;
private static class B {
int fieldOfB;
}
public static void main(String[] args) {
B obj = new B() { //this anonymous class is static becuase is in the main method.
private static void testMethod() { //so why here i have an error and i can put just a non-static method
//if my class is static ?
//a class static can have static method, but this class no, why?
}
};
}
}
it's sure that anonymous class are static?
An anonymous class is static if the context is static. e.g. in a static method.
An anonymous class is non static if there is a non static context, whether you need it to be non-static or not. The compiler is not smart enough to make a class static if the non static context is not used.
In this example, two anonymous classes were created. One in a static method has no reference to an outer class and is like a static nested class.
Note: these classes are still called "Inner" and cannot have static members even though they have no reference to an Outer class.
import java.util.Arrays;
public class Main {
Object o = new Object() {
{
Object m = Main.this; // o has a reference to an outer class.
}
};
static Object O = new Object() {
// no reference to Main.this;
// doesn't compile if you use Math.this
};
public void nonStaticMethod() {
Object o = new Object() {
{
Object m = Main.this; // o has a reference to an outer class.
}
};
printFields("Anonymous class in nonStaticMethod", o);
}
public static void staticMethod() {
Object o = new Object() {
// no reference to Main.this;
// doesn't compile if you use Math.this
};
printFields("Anonymous class in staticMethod", o);
}
private static void printFields(String s, Object o) {
System.out.println(s + " has fields " + Arrays.toString(o.getClass().getDeclaredFields()));
}
public static void main(String... ignored) {
printFields("Non static field ", new Main().o);
printFields("static field ", Main.O);
new Main().nonStaticMethod();
Main.staticMethod();
}
}
prints
Non static field has fields [final Main Main$1.this$0]
static field has fields []
Anonymous class in nonStaticMethod has fields [final Main Main$3.this$0]
Anonymous class in staticMethod has fields []
From JLS 15.9.5:
An anonymous class is always an inner class (§8.1.3); it is never static (§8.1.1, §8.5.1).
Section 8.1.3 talks more about inner classes, including when they occur in a static context. But they're never static themselves, and thus can't declare static members (other than constant variables).
