從認識java的那天起,就被告知String是不可變的,因爲源碼上是這樣寫的
public final class String
implements java.io.Serializable, Comparable<String>, CharSequence {
/** The value is used for character storage. */
private final char value[];
/** Cache the hash code for the string */
private int hash; // Default to 0
/** use serialVersionUID from JDK 1.0.2 for interoperability */
private static final long serialVersionUID = -6849794470754667710L;
……
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很好理解,因爲被final關鍵字修飾了,所以是不可變的,但你能清楚的解釋下面的問題嗎
public static void main(String[] args) {
String a = new String("abcd");
String b = new String("abcd");
String c = "abcd" + "ppp";
String d = "abcd";
String e = "abcd" + "ppp";
String f = d + "ppp";
System.out.println("情況1:"+(a == b) + "-------------" + a.equals(b));
System.out.println("情況2:"+(a == d) + "-------------" + a.equals(d));
System.out.println("情況4:"+(c == e) + "-------------" + c.equals(e));
System.out.println("情況5:"+(f == e) + "-------------" + f.equals(e));
a = a.intern();
System.out.println("情況6:"+(a == d) + "-------------" + a.equals(d));
}
是不是瘋了?
其實每種情況的後半段equals
好理解,根據源碼的描述
public boolean equals(Object anObject) {
if (this == anObject) {
return true;
}
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;
}
看上面的源碼知道String重寫的Object的eqals方法,本質是轉成字符數組然後逐一比較,所以上述6種情況後半段都是true
一個個來分析,情況1
String a = new String("abcd");
String b = new String("abcd");
System.out.println("情況1:"+(a == b) + "-------------" + a.equals(b));
看一下帶參數的構造函數的源碼:
/**
* Initializes a newly created {@code String} object so that it represents
* the same sequence of characters as the argument; in other words, the
* newly created string is a copy of the argument string. Unless an
* explicit copy of {@code original} is needed, use of this constructor is
* unnecessary since Strings are immutable.
*
* @param original
* A {@code String}
*/
public String(String original) {
this.value = original.value;
this.hash = original.hash;
}
首先this.value
是String定義的一個私有的被final
修飾成員變量private final char value[];
,將原始字符串的value和hash值進行賦值後,會返回一個String對象的引用,所以a和b返回的是兩個引用,自然不一樣,所以情況1前半段爲false
情況2
String a = new String("abcd");
String d = "abcd";
System.out.println("情況2:"+(a == d) + "-------------" + a.equals(d));
看着好像是一樣的,但這個就涉及到String字符池的概念,其實在情況1中,jvm會在內部維護的String Pooll中放入一個"abc"對象,並在heap中創建一個String對象,然後將該heap中對象的引用返回給用戶
大概意思如下圖
堆內存和字符串有什麼不同呢,堆內存會隨着對象的回收而被GC回收,而字符池不會
所以執行String d = "abcd";
的時候,實際上java的處理方式是先去字符池找,有沒有已經存在的字符串,如果有,則返回一個指向它的引用,如果沒有,則與new String(“abcd”)處理一樣,這裏可知,字符池中已經有,所以返回的只是一個String類型的引用,而===
比較的就是引用,所以情況2的前半段結果也是false
情況3
String b = new String("abcd");
System.out.println("情況3:"+(b == d) + "-------------" + b.equals(d));
String d = "abcd";
與情況2如出一轍,前半段結果也是false
情況4
String c = "abcd" + "ppp";
String e = "abcd" + "ppp";
System.out.println("情況4:"+(c == e) + "-------------" + c.equals(e));
如果用+號來實現String的串接時:1)僅當+號兩邊均爲字符串常量時,纔將其+後的結果當做字符串常量,且該結果直接放入String Pool;2)若+號兩邊有一方爲變量時,+後的結果即當做非字符串常量處理(等同於new String()的效果) ,所以可知c和e引用的是用一個對象,所以情況4的前半段結果是true
情況5
String d = "abcd";
String e = "abcd" + "ppp";
String f = d + "ppp";
System.out.println("情況5:"+(f == e) + "-------------" + f.equals(e));
根據情況4的分析,+號有一方爲變量,處理與new String()
一致,自然前半段結果也是false
情況6
String a = new String("abcd");
a = a.intern();
System.out.println("情況6:"+(a == d) + "-------------" + a.equals(d));
情況6與情況2唯一的不同就是a = a.intern();
,那這個a = a.intern();
到底做了什麼呢,源碼是這樣說的
/**
* Returns a canonical representation for the string object.
* <p>
* A pool of strings, initially empty, is maintained privately by the
* class {@code String}.
* <p>
* When the intern method is invoked, if the pool already contains a
* string equal to this {@code String} object as determined by
* the {@link #equals(Object)} method, then the string from the pool is
* returned. Otherwise, this {@code String} object is added to the
* pool and a reference to this {@code String} object is returned.
* <p>
* It follows that for any two strings {@code s} and {@code t},
* {@code s.intern() == t.intern()} is {@code true}
* if and only if {@code s.equals(t)} is {@code true}.
* <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();
大概意思就是如果常量池中存在當前字符串, 就會直接返回當前字符串. 如果常量池中沒有此字符串, 會將此字符串放入常量池中後, 再返回
所以a.intern()
是顯式的調用了intern()
方法,將a的引用由原來的指向heap中對象改爲指向內部維護的strings pool中的對像,a原來指向的String對象已經成爲垃圾對象了,隨時會被GC收集
如此一來自然前半段結果就是true
了,真TM神奇
總結一下,String對象真的是不可變的,“可變的”是引用,jvm通過上面的策略,可以使多個引用指向一個對象而互不影響。字符池的存在當然是基於節約內存考慮。
最後我們來看一下結果
好像是全對了,100分,但是故事結束了嗎?
public static String changeStr(String before) throws Exception{
System.out.println("之前是這個---- "+before); //
Field field = String.class.getDeclaredField("value");
field.setAccessible(true);
char[] value = (char[]) field.get(before);
value[6] = 'J';
value[7] = 'a';
value[8] = 'v';
value[9] = 'a';
value[10] = '!';
value[11] = '!';
System.out.println("之後呀是這個---- "+before);
return before;
}
public static void main(String[] args) {
try {
System.out.println(changeStr("Hello String"));
}catch (Exception e ){
// 異常處理
}
}
啪,打臉,好疼!!!