java 与 c#的 中 字符串比较“==”与“equals”的差异
.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();
}
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
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
}
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 }
