概述

本类文章会不段更新分析学习到的经典面试题目，在此记录下来便于自己理解。如果有不对的地方还请各位观众拍砖。 

今天主要分享一下常用的字符串的几个题目，相信学习java的小伙伴们对String类是再熟悉不过了，今天我们就来和她再次邂逅，好了下面开始。

先来说说String特点

String是不可变的常量，每当我们创建一个字符串对象的时候，如果堆区的常量池里不存在这个字符串，就会创建一个存储在常量池里(String存的地方叫String pool)，如果存在了，就直接把变量的地址指向常量池里，比如：String b = “abc” 这句话 内存表示如下。下面开始上题 

1.1

String s1 = new String("abc");String s2 = new String("abc"); System.out.println(s1 == s2);

 

输出结果是什么呢？ 

从上面的图也大概说了jvm里面有堆、栈区。栈区里面主要存放的是局部变量，堆区存放的是new出来的对象。==对于对象类型比较的是地址。所以在s1和s1是分别引用了堆里面new出来的不同对象的地址，图形理解如下

答案很明显了，地址不同 输出false.

1.2

String s1 = "abc";StringBuffer s2 = new StringBuffer(s1); System.out.println(s1.equals(s2));

这是true 还是false呢？答案是false。

首先s1变量引用了字符串”abc”,然后StringBuffer s2 = new StringBuffer(s1)，新建了一个StringBuffer对象调用append()方法返回自身。调用String的equals方法。重点就是这个equals方法里有个instance of，必需是同一类型的才进行比较否则直接返回false。 

来看一下源码:

/**     * Compares this string to the specified object.  The result is {@code     * true} if and only if the argument is not {@code null} and is a {@code     * String} object that represents the same sequence of characters as this     * object.     *     * @param  anObject     *         The object to compare this {@code String} against     *     * @return  {@code true} if the given object represents a {@code String}     *          equivalent to this string, {@code false} otherwise     *     * @see  #compareTo(String)     * @see  #equalsIgnoreCase(String)     */    public boolean equals(Object anObject) {        if (this == anObject) {            return true;        }        //关键点就在这里了        if (anObject instanceof String) {            String anotherString = (String) anObject;            int n = value.length; if (n == anotherString.value.length) { char v1[] = value; char v2[] = anotherString.value; int i = 0; while (n-- != 0) { if (v1[i] != v2[i]) return false; i++; } return true; } } return false; }

 

1.3

下面的代码在内存会产生几个对象呢？ 

String s1 = new String(“abc”); String s2 = new String(“abc”);

 

答案：3个 

有了上面的分析，相信大家都明白了，new了两个对象，加上string pool里的一个”abc”。

1.4

下面的代码输出结果是啥？

String s1 = "abc";String s2 = new String("abc");s2.intern();System.out.println(s1 ==s2);

 

我们可能对intern()这个方法不太熟悉，先来看看注释：

/**     * 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 {@link #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.     *     * @return  a string that has the same contents as this string, but is     *          guaranteed to be from a pool of unique strings.     */    public native String intern();

 

注释好多我草，关键的是这句：

When the intern method is invoked, if the pool already contains a string equal to this String object as determined by the {@link #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.

大致就是说，如果常量池里不存在这个字符串，就创建一个并且返回地址，否则的话直接返回地址。

上面的代码第二行String s2 = new String(“abc”); s2其实是引用到了new的对象，虽然在第三行调用了intern方法，但是没有赋值给s2，所以s2的引用还是没有变。所以返回false。 

如果第三行代码改成s2 = s2.intern()就会返回true了。

String s1 = "abc"; String s2 = new String("abc"); s2 = s2.intern(); System.out.println(s1==s2);

 

好了，今天就到这里。之后会继续分析。如果喜欢我的文章欢迎关注我。求赞，有问题可以评论，各位的支持是我最大的动力！！