String问题求解

求大佬帮忙解释下为什么 

        String s1 = "Programming";
        String s2 = new String("Programming");
        String s3 = "Program";
        String s4 = "ming";
        String s5 = "Program" + "ming";
        String s6 = s3 + s4;
        System.out.println(s1 == s5);//true
        System.out.println(s1 == s6);//false
        System.out.println(s5 == s6);//false