java - 编译时的简单 Java 错误代码

Question

我正在尝试运行以下代码。它给我错误

The operator + is undefined for the argument type(s) String, void

public class CompStrings{

    String s1=null;
    String s2=null;

    public void testMethod(){


        s1 = new String("Hello");
        s2 = new String("Hello");
        String str3="abc";
        String  str4 ="abc";
        /**
         * ==
         */
        if(str3==str4){
            System.out.println("str3==str4 is true");
        }else{
            System.out.println("str3==str4 is false");
        }

        if(str3.equals(str4)){
            System.out.println("str1.equals(str2) is true");
        }else{
            System.out.println("str1.equals(str2) is false");
        }

        System.out.println(s1.hashCode());
        System.out.println(s2.hashCode());

        System.out.println(s1 + " equals " + s2 + " -> " +
        s1.equals(s2));

        System.out.println(s1 + " == " + s2 + " -> " + (s1 == s2)); 
        /*Integer i1 = new Integer(10);
        Integer i2 = new Integer(10);
        System.out.println(i1.hashCode());
        System.out.println(i2.hashCode());
        1)String s1="hello";
        String s2="hello";

        2)String s1 = new String("Hello");
        String s2 = new String("Hello");

        3)String s1="hello";
        string s2=s1;

        **/

    }
    public static void main(String argv[]){
        CompStrings obj= new CompStrings();
//      \System.out.println("Calling My Method"+ obj.testMethod());

    System.out.println("Hashcode for emp1 = " + obj.testMethod());// Here it gives Error



    }


    public boolean equals(Object o){
        String s = (String) o;
        if (s1.equals(s) ){
            return true;
        }else{
            return false;
        }


        }

        public int hashCode(){
        return s1.hashCode();

        }

}

Answer 1

testMethod() 显然不返回任何内容，这就是没有任何内容可打印的原因。