本類文章會不段更新分析學習到的經典面試題目，在此記錄下來便於自己理解。如果有不對的地方還請各位觀眾拍磚。
今天主要分享一下常用的字符串的幾個題目，相信學習java的小伙伴們對String類是再熟悉不過了，今天我們就來和她再次邂逅，好了下面開始。
String是不可變的常量，每當我們創建一個字符串對象的時候，如果堆區的常量池裡不存在這個字符串，就會創建一個存儲在常量池裡(String存的地方叫String pool)，如果存在了，就直接把變量的地址指向常量池裡，比如：String b = “abc” 這句話 內存表示如下。下面開始上題
String s1 = new String("abc");
String s2 = new String("abc");
System.out.println(s1 == s2);
輸出結果是什麼呢？
從上面的圖也大概說了jvm裡面有堆、棧區。堆區裡面主要存放的是局部變量，棧區裡存放的是new出來的對象。==對於對象類型比較的是地址。所以在s1和s1是分別引用了堆裡面new出來的不同對象的地址，圖形理解如下
答案很明顯了，地址不同 輸出false.
String s1 = "abc"; StringBuffer s2 = new StringBuffer(s1); System.out.println(s1.equals(s2));
這是true 還是false呢？答案是false。
首先s1變量引用了字符串”abc”,然後StringBuffer s2 = new StringBuffer(s1)，新建了一個StringBuffer對象調用append()方法返回自身。調用String的equals方法。重點就是這個equals方法裡有個instance of，必需是同一類型的才進行比較否則直接返回false。
來看一下源碼:
/**
* 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;
}
//關鍵點就在這裡了
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
下面的代碼在內存會產生幾個對象呢？
String s1 = new String(“abc”); String s2 = new String(“abc”);
答案：3個
有了上面的分析，相信大家都明白了，new了兩個對象，加上string pool裡的一個”abc”。
下面的代碼輸出結果是啥？
String s1 = "abc";
String s2 = new String("abc");
s2.intern();
System.out.println(s1 ==s2);
我們可能對intern()這個方法不太熟悉，先來看看注釋：
/**
* Returns a canonical representation for the string object.
* <p>
* A pool of strings, initially empty, is maintained privately by the
* class <code>String</code>.
* <p>
* When the intern method is invoked, if the pool already contains a
* string equal to this <code>String</code> object as determined by
* the {@link #equals(Object)} method, then the string from the pool is
* returned. Otherwise, this <code>String</code> object is added to the
* pool and a reference to this <code>String</code> object is returned.
* <p>
* It follows that for any two strings <code>s</code> and <code>t</code>,
* <code>s.intern() == t.intern()</code> is <code>true</code>
* if and only if <code>s.equals(t)</code> is <code>true</code>.
* <p>
* All literal strings and string-valued constant expressions are
* interned. String literals are defined in section 3.10.5 of the
* <cite>The Java™ Language Specification</cite>.
*
* @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();
注釋好多我草，關鍵的是這句：
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.
大致就是說，如果常量池裡不存在這個字符串，就創建一個並且返回地址，否則的話直接返回地址。
上面的代碼第二行String s2 = new String(“abc”); s2其實是引用到了new的對象，雖然在第三行調用了intern方法，但是沒有賦值給s2，所以s2的引用還是沒有變。所以返回false。
如果第三行代碼改成s2 = s2.intern()就會返回true了。
String s1 = "abc";
String s2 = new String("abc");
s2 = s2.intern();
System.out.println(s1==s2);
好了，今天就到這裡。之後會繼續分析。如果喜歡我的文章歡迎關注我。求贊，有問題可以評論，各位的支持是我最大的動力！！
