Java中String比较:==、equals()方法、intern()方法

系统管理员 2022-05-27 10:09 176阅读 0赞

*   *  Java中String比较:==、equals()方法、intern()方法
        
         *  一、 ==比较
            
             *  1.01 简单类型
             *  1.02 引用类型
             *  1.03 String的==比较
         *  二、 euqals()比较
            
             *  2.01 String中的重写的equals()方法:
         *  三、 intern方法介绍

## Java中String比较:==、equals()方法、intern()方法 ##

涉及到String的操作==,equals,intern方法。

\*文中出现的代码基于JDK1.8\_144版本.

### 一、 ==比较 ###

#### 1.01 简单类型 ####

\{ byte, boolean, short, char, int, long, float, double\}

直接**比较值**。

#### 1.02 引用类型 ####

比较**地址值**，如果指向同一个地址，返回true。否则返回false。

#### 1.03 String的==比较 ####

可以对比**2.01 String中的重写的equals()方法:**来看。

废话不多说，直接上代码。

/** * . * ==判断引用是否相等. */
        public static void testEquals2() {
            String a = "a";
            final String c = "a";
    
            String b = a + "b";
            String d = c + "b";
            String e = getA() + "b";
    
            String compare = "ab";
    
            System.out.println(b == compare);
            System.out.println(d == compare);//编译优化，final能确定，所以指向同一个
            System.out.println(e == compare);
        }
    
        /** * 测试用. * @return “a” */
        private static String getA() {
            return "a";
        }

由于编译优化机制，能在编译时确定的直接优化到常量池中。所以相等。如fianl。

-----testEquals2()-----
    false
    true
    false

### 二、 euqals()比较 ###

Object中的equals()方法 与 == 相同。

#### 2.01 String中的重写的equals()方法: ####

Compares this string to the specified object. The result is true if and only if the argument is not null and is a String object that represents the same sequence of characters as this object.

比较他们的字符串值是否相同。

public static void testEquals3() {
            String a = "a";
            final String c = "a";
    
            String b = a + "b";
            String d = c + "b";
            String e = getA() + "b";
    
            String compare = "ab";
    
            System.out.println(b.equals(compare));
            System.out.println(d.equals(compare));
            System.out.println(e.equals(compare));
        }

-----testEquals3()-----
    true
    true
    true

毫无疑问相同，我们来看看JDK源码吧:

JDK8源码

public boolean equals(Object var1) {
            if (this == var1) {
                return true;
            } else {
                if (var1 instanceof String) {
                    String var2 = (String)var1;
                    int var3 = this.value.length;
                    if (var3 == var2.value.length) {
                        char[] var4 = this.value;
                        char[] var5 = var2.value;
    
                        for(int var6 = 0; var3-- != 0; ++var6) {
                            if (var4[var6] != var5[var6]) {
                                return false;
                            }
                        }
    
                        return true;
                    }
                }
    
                return false;
            }
        }

\*当然，String也重写了hashCode()方法。

### 三、 intern方法介绍 ###

Returns a canonical representation for the string object.  
A pool of strings, initially empty, is maintained privately by the class String.

When the intern method is invoked, if the pool already contains a string equal to this String object as determined by the equals(Object) method, then the string from the pool is returned. Otherwise, this String object is added to the pool and a reference to this String object is returned.

It follows that for any two strings s and t, s.intern() == t.intern() is true if and only if s.equals(t) is true.

All literal strings and string-valued constant expressions are interned. String literals are defined in section 3.10.5 of the The Java™ Language Specification.

intern()方法:  
当调用intern方法时，JVM会在常量池中通过equals()方法来匹配是否存在等值的String,  
如果存在，则直接返回其地址。否则再新建.  
所以，只要值相等，用.intern()==.intern()判断一定相等.  
但是intern效率不高.

private static void testIntern() {
            String a = "a";
            String b = a + "b";
            String c = "ab";
            String d = new String(b);
    
            System.out.println(b == c);
            System.out.println(c == d);
            System.out.println(c == d.intern());
            System.out.println(b.intern() == d.intern());
        }

-----testIntern()-----
    false
    false
    true
    true

--------------------

参考资料:

[Java 8 API][]  
< Java特种兵(上册) > 谢宇 ISBN: 9787121239359

[Java 8 API]: https://docs.oracle.com/javase/8/docs/api/