java学习：字符串比较“==”与“equals”的差异及与c#的区别

.net中，其字符串特有的驻留机制，保证了在同一进程中，相同字符序列的字符串，只有一个实例，这样能避免相同内容的字符串重复实例化，以减少性能开销。

先来回顾一下c#中的代码：

        public static void testString()
        {
            String s = "Abc";
            String s1 = "abc";
            String s2 = "abc";


            Console.WriteLine("s1==s2 ? " + (s1 == s2)); //true
            Console.WriteLine("s1.Equals(s2) ? " + s1.Equals(s2)); //true
            Console.WriteLine("String.Compare(s1,s,true) ? " + String.Compare(s1, s, true)); //0
            Console.WriteLine("------------------------");


            char[] chr = { 'a', 'b', 'c' };
            String s3 = new String(chr);
            Console.WriteLine("s1==s3 ? " + (s1 == s3)); //true
            Console.WriteLine("s1.equals(s3) ? " + s1.Equals(s3)); //true
            Console.WriteLine("String.Compare(s3, s, true) ? " + String.Compare(s3, s, true)); //0
            Console.WriteLine("------------------------");

            String t = "bc";
            String s4 = "a" + t;
            Console.WriteLine("s1==s4 ? " + (s1 == s4)); //true
            Console.WriteLine("s1.equals(s4) ? " + s1.Equals(s4)); //true
            Console.WriteLine("String.Compare(s4, s, true) ? " + String.Compare(s4, s, true)); //0
            Console.WriteLine("------------------------");

            String s5 = "a" + "bc";
            Console.WriteLine("s1==s5 ? " + (s1 == s5)); //true
            Console.WriteLine("s1.equals(s5) ? " + s1.Equals(s5)); //true
            Console.WriteLine("String.Compare(s5, s, true) ? " + String.Compare(s5, s, true)); //0


            Console.Read();


        }

从运行结果可以看出，无论你怎么折腾，只要二个字符串的内容完全相同，引用始终只有一个。

java中其实也有类似的机制，称为“字符串常量池”，但是java中却允许 用new String(String str)的方式创建多个相同内容的实例。为了能区别这二种情况，java中的==与equals用来判断字符串是否相等时，赋予了不同的含义。

==用于判定二个字符串是否引用相同，而equals用于判断二个字符串是否内容相同

public static void testString(){
        String s = "Abc";
        String s1 = "abc";
        String s2 = "abc";
        
        System.out.println("s1==s2 ? " + (s1==s2)); //true
        System.out.println("s1.equals(s2) ? " + s1.equals(s2)); //true
        System.out.println("s1.equalsIgnoreCase(s) ? " + s1.equalsIgnoreCase(s)); //true
        System.out.println("------------------------");
        
        String s3 = new String("abc"); 
        System.out.println("s1==s3 ? " + (s1==s3)); //false
        System.out.println("s1.equals(s3) ? " + s1.equals(s3)); //true    
        System.out.println("s3.equalsIgnoreCase(s) ? " + s3.equalsIgnoreCase(s)); //true
        System.out.println("------------------------");
        
        char[] chr ={'a','b','c'};
        String s4 = new String(chr);
        System.out.println("s1==s4 ? " + (s1==s4)); //false
        System.out.println("s1.equals(s4) ? " + s1.equals(s4)); //true
        System.out.println("s4.equalsIgnoreCase(s) ? " + s4.equalsIgnoreCase(s)); //true
        System.out.println("------------------------");
        
        String t ="bc";
        String s5 = "a" + t;
        System.out.println("s1==s5 ? " + (s1==s5)); //false
        System.out.println("s1.equals(s5) ? " + s1.equals(s5)); //true
        System.out.println("s5.equalsIgnoreCase(s) ? " + s5.equalsIgnoreCase(s)); //true
        System.out.println("------------------------");
        
        String s6 = "a" + "bc";
        System.out.println("s1==s6 ? " + (s1==s6)); //true
        System.out.println("s1.equals(s6) ? " + s1.equals(s6)); //true
        System.out.println("s6.equalsIgnoreCase(s) ? " + s6.equalsIgnoreCase(s)); //true
        
        
    }