String StringBuilder和StringBuffer 结合源码解析

 1 String s = "abc";
 2 char data[] = {'a', 'b', 'c'};
 3 String str = new String(data);
 4 System.out.println(str);//abc
 5 System.out.println(str.length());//长度为3其实，String的内部是个数组，
 6 //源码如下：
 7 /**
 8  * Allocates a new {@code String} so that it represents the sequence of
 9  * characters currently contained in the character array argument. The
10  * contents of the character array are copied; subsequent modification of
11  * the character array does not affect the newly created string.
12  *
13  * @param  value
14  *         The initial value of the string
15  */
16 public String(char value[]) {
17     this.value = Arrays.copyOf(value, value.length);
18 }
19 //那么为什么String类型的变量是不可变的呢？从源码中解析：
20 /** The value is used for character storage. */
21 private final char value[];
22 
23 /** Cache the hash code for the string */
24 private int hash; // Default to 0
25 //这两个属性其中char是final的，也是不可变的，private类型，其中，在String中没有两//个属性的get、set方法，既无法去设置String的值，因此是无法改变的。

 1 public final class StringBuilder
 2     extends AbstractStringBuilder
 3     implements java.io.Serializable, CharSequence{}//你会发现fianl的，既不可继承的。
 4 /**
 5  * Constructs a string builder with no characters in it and an
 6  * initial capacity of 16 characters.
 7  */
 8 public StringBuilder() {
 9     super(16);
10 }//当你创建了一个StringBuilder的时候，其长度默认为16。
11 /**
12  * Constructs a string builder initialized to the contents of the
13  * specified string. The initial capacity of the string builder is
14  * <code>16</code> plus the length of the string argument.
15  *
16  * @param   str   the initial contents of the buffer.
17  * @throws    NullPointerException if <code>str</code> is <code>null</code>
18  */
19 public StringBuilder(String str) {
20     super(str.length() + 16);
21     append(str);
22 }
23 //如果长度大于了16,StringBuilder会在16的基础上进行扩容。/**
24  * Constructs a string builder with no characters in it and an
25  * initial capacity specified by the <code>capacity</code> argument.
26  *
27  * @param      capacity  the initial capacity.
28  * @throws     NegativeArraySizeException  if the <code>capacity</code>
29  *               argument is less than <code>0</code>.
30  */
31 public StringBuilder(int capacity) {
32     super(capacity);
33 }
34 //你也可以自定义长度，但是长度不能小于0，否则会NegativeArraySizeException异常。    public StringBuilder append(boolean b) {
35     super.append(b);
36     return this;
37 }
38 
39 public StringBuilder append(char c) {
40     super.append(c);
41     return this;
42 }
43 
44 public StringBuilder append(int i) {
45     super.append(i);
46     return this;
47 }
48 
49 public StringBuilder append(long lng) {
50     super.append(lng);
51     return this;
52 }
53 
54 public StringBuilder append(float f) {
55     super.append(f);
56     return this;
57 }
58 
59 public StringBuilder append(double d) {
60     super.append(d);
61     return this;
62 }

public final class StringBuffer
   extends AbstractStringBuilder
   implements java.io.Serializable, CharSequence{}

 1 public synchronized StringBuffer append(Object obj) {
 2     super.append(String.valueOf(obj));
 3     return this;
 4 }
 5 
 6 public synchronized StringBuffer append(String str) {
 7     super.append(str);
 8     return this;
 9 }public synchronized StringBuffer append(boolean b) {
10     super.append(b);
11     return this;
12 }
13 
14 public synchronized StringBuffer append(char c) {
15     super.append(c);
16     return this;
17 }
18 
19 public synchronized StringBuffer append(int i) {
20     super.append(i);
21     return this;
22 }