String s1 = null; String s2 = null; System.out.println(s1==s2);//true //System.out.println(s1.equals(s2));//NullPointerException s1 = s2; System.out.println(s1==s2);//true //System.out.println(s1.equals(s2));//NullPointerException System.out.println("***1***");
s1 = null; s2 = ""; System.out.println(s1==s2);//false //System.out.println(s1.equals(s2));//NullPointerException s1 = s2; System.out.println(s1==s2);//true System.out.println(s1.equals(s2));//true System.out.println("***2***");
s1 = ""; s2 = null; System.out.println(s1==s2);//false System.out.println(s1.equals(s2));//false s1 = s2; System.out.println(s1==s2);//true //System.out.println(s1.equals(s2));//NullPointerException System.out.println("***3***");
s1 = ""; s2 = ""; System.out.println(s1==s2);//true System.out.println(s1.equals(s2));//true s1 = s2; System.out.println(s1==s2);//true System.out.println(s1.equals(s2));//true System.out.println("***4***"); s1 = new String(""); s2 = new String(""); System.out.println(s1==s2);//false System.out.println(s1.equals(s2));//true s1 = s2; System.out.println(s1==s2);//true System.out.println(s1.equals(s2));//true System.out.println("***5***");
s1 = "null"; s2 = "null"; System.out.println(s1==s2);//ture System.out.println(s1.equals(s2));//true s1 = s2; System.out.println(s1==s2);//true System.out.println(s1.equals(s2));//true System.out.println("***6***"); s1 = new String("null"); s2 = new String("null"); System.out.println(s1==s2);//flase System.out.println(s1.equals(s2));//true s1 = s2; System.out.println(s1==s2);//true System.out.println(s1.equals(s2));//true System.out.println("***7***");
s1 = "abc"; s2 = "abc"; System.out.println(s1==s2);//ture System.out.println(s1.equals(s2));//true s1 = s2; System.out.println(s1==s2);//true System.out.println(s1.equals(s2));//true System.out.println("***8***"); s1 = new String("abc"); s2 = new String("abc"); System.out.println(s1==s2);//false System.out.println(s1.equals(s2));//true s1 = s2; System.out.println(s1==s2);//true System.out.println(s1.equals(s2));//true System.out.println("***9***");
总结:
凡是比较基本类型只能用==,比较结果你看到的字面值相等就会相等,
什么叫基本类型? byte,short,int,long,char,float,double这几种就是基本类型.基本类型不是对象所以不存在用equals比较. 凡是比较引用类型(对象),==比较的是两个引用是不是指向同一个内存地址,equals比较的是两个引用的字面值是不是相同,什么叫字面值?你能看到的值就是字面值,比如:String s="abc"; abc就是字面值. 基本类型的包装类型是对象类型,所以除非两个相同字面值的引用指向同一内存地址,值才会相等,比如: Integer a=new Integer(3);Integer b=a;//互相赋值这样用等号或equals比较都会得出true,记住一点,凡是两个引用都用了new去初始化,那==比较的结果都只会是false,互相之间有了赋值,比较结果就为true. 比较不好理解的一点: String a="abc"; String b="abc"; a==b和a.equals(b);结果都会为true,因为没有用new去新建对象,其实a和b都指向的是同一个String对象"abc",改成: String a=new String("abc"); String b=new String"abc"); 后==的结果就是false了.总之参照上面的说明,不难理解.
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