Java: why Thread.sleep() and yield() are static?

Question

Why sleep() and yield() methods are defined as static methods in java.lang.Thread class?

Answer 1

Both sleep and yield methods are native. To understand better from above answers I made two classes ClassA and ClassB with same static method. I invoked method of other class to check its behavior. So we can call other class' static method.
So may be there is other reason behind making sleep method static.

public class ClassA {

    public static void method(){
        System.out.println("Inside ClassA method");
    }

    public static void main(String[] args) {
        method();
        ClassB classb = new ClassB();
        classb.method();
    }
}

    public class ClassB {
    public static void method(){
        System.out.println("Inside ClassB method");
    } 

}