.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：
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1 public static void testString() 2 { 3 String s = "Abc"; 4 String s1 = "abc"; 5 String s2 = "abc"; 6 7 8 Console.WriteLine("s1==s2 ? " + (s1 == s2)); //true 9 Console.WriteLine("s1.Equals(s2) ? " + s1.Equals(s2)); //true 10 Console.WriteLine("String.Compare(s1,s,true) ? " + String.Compare(s1, s, true)); //0 11 Console.WriteLine("------------------------"); 12 13 14 char[] chr = { 'a', 'b', 'c' }; 15 String s3 = new String(chr); 16 Console.WriteLine("s1==s3 ? " + (s1 == s3)); //true 17 Console.WriteLine("s1.equals(s3) ? " + s1.Equals(s3)); //true 18 Console.WriteLine("String.Compare(s3, s, true) ? " + String.Compare(s3, s, true)); //0 19 Console.WriteLine("------------------------"); 20 21 String t = "bc"; 22 String s4 = "a" + t; 23 Console.WriteLine("s1==s4 ? " + (s1 == s4)); //true 24 Console.WriteLine("s1.equals(s4) ? " + s1.Equals(s4)); //true 25 Console.WriteLine("String.Compare(s4, s, true) ? " + String.Compare(s4, s, true)); //0 26 Console.WriteLine("------------------------"); 27 28 String s5 = "a" + "bc"; 29 Console.WriteLine("s1==s5 ? " + (s1 == s5)); //true 30 Console.WriteLine("s1.equals(s5) ? " + s1.Equals(s5)); //true 31 Console.WriteLine("String.Compare(s5, s, true) ? " + String.Compare(s5, s, true)); //0 32 33 34 Console.Read(); 35 36 37 }
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
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1 public static void testString(){ 2 String s = "Abc"; 3 String s1 = "abc"; 4 String s2 = "abc"; 5 6 System.out.println("s1==s2 ? " + (s1==s2)); //true 7 System.out.println("s1.equals(s2) ? " + s1.equals(s2)); //true 8 System.out.println("s1.equalsIgnoreCase(s) ? " + s1.equalsIgnoreCase(s)); //true 9 System.out.println("------------------------"); 10 11 String s3 = new String("abc"); 12 System.out.println("s1==s3 ? " + (s1==s3)); //false 13 System.out.println("s1.equals(s3) ? " + s1.equals(s3)); //true 14 System.out.println("s3.equalsIgnoreCase(s) ? " + s3.equalsIgnoreCase(s)); //true 15 System.out.println("------------------------"); 16 17 char[] chr ={'a','b','c'}; 18 String s4 = new String(chr); 19 System.out.println("s1==s4 ? " + (s1==s4)); //false 20 System.out.println("s1.equals(s4) ? " + s1.equals(s4)); //true 21 System.out.println("s4.equalsIgnoreCase(s) ? " + s4.equalsIgnoreCase(s)); //true 22 System.out.println("------------------------"); 23 24 String t ="bc"; 25 String s5 = "a" + t; 26 System.out.println("s1==s5 ? " + (s1==s5)); //false 27 System.out.println("s1.equals(s5) ? " + s1.equals(s5)); //true 28 System.out.println("s5.equalsIgnoreCase(s) ? " + s5.equalsIgnoreCase(s)); //true 29 System.out.println("------------------------"); 30 31 String s6 = "a" + "bc"; 32 System.out.println("s1==s6 ? " + (s1==s6)); //true 33 System.out.println("s1.equals(s6) ? " + s1.equals(s6)); //true 34 System.out.println("s6.equalsIgnoreCase(s) ? " + s6.equalsIgnoreCase(s)); //true 35 36 37 }
