实例化对象时编译错误以及运行时抛出空指针异常状况

public class Test {
    public static void main(String[] args) {
       //实例化对象时，只写右边，直接编译错误：Variable 'a' might not have been initialized
        NewQuestion a;
        System.out.println(a.testMethon());
        //实例化对象时，=null情况，运行时抛出异常:抛出 NullPointerException 异常
        NewQuestion b=null;
        System.out.println(b.testMethon());
        //正确的实例化方式 打印true 0 null
        NewQuestion c=new NewQuestion();
        System.out.println(c.testMethon()+"\t");
        //成员变量不写会有默认值
    }
}

package newqueastion;

public class NewQuestion {
    int a;
    int[] b=null;
    public boolean testMethon(){
        return true;
    }
}