AbstractStringBuilder

2017-01-29

String是不变字符串，而要实现可变字符串，可以通过继承AbstractStringBuilder类，AbstractStringBuilder是抽象类，实现了Appendable，CharSequence接口，具有append能力，同时也具备了返回字符串长度，返回特定位置字符，返回子串等能力，是StringBuilder和StringBuffer的父类。

类定义:

abstract class AbstractStringBuilder implements Appendable, CharSequence

从类定义看，AbstractStringBuilder是包私有。

属性:

私有属性:

char value[];（value数组保存字符串，不是final类型）

int count; 字符串的长度

方法:

AbstractStringBuilder提供了2个构造函数：

//默认构造函数。
AbstractStringBuilder() {
}

//以capacity参数创建保存字符串的数组。
AbstractStringBuilder(int capacity) {
    value = new char[capacity];
}

append()函数系列

public AbstractStringBuilder append(Object obj)
public AbstractStringBuilder append(String str)
public AbstractStringBuilder append(StringBuffer sb)
public AbstractStringBuilder append(CharSequence s)
public AbstractStringBuilder append(CharSequence s, int start, int end)
public AbstractStringBuilder append(char[] str)
public AbstractStringBuilder append(char str[], int offset, int len)
public AbstractStringBuilder append(boolean b)
public AbstractStringBuilder append(char c)
public AbstractStringBuilder append(int i)
public AbstractStringBuilder append(long l)
public AbstractStringBuilder append(float f)
public AbstractStringBuilder append(double d)

insert()函数系列

public AbstractStringBuilder insert(int index, char[] str, int offset,int len)
public AbstractStringBuilder insert(int offset, Object obj)
public AbstractStringBuilder insert(int offset, String str)
public AbstractStringBuilder insert(int offset, char[] str)
public AbstractStringBuilder insert(int dstOffset, CharSequence s
public AbstractStringBuilder insert(int dstOffset, CharSequence s,int start, int end)
public AbstractStringBuilder insert(int offset, boolean b)
public AbstractStringBuilder insert(int offset, char c)
public AbstractStringBuilder insert(int offset, int i)
public AbstractStringBuilder insert(int offset, long l)
public AbstractStringBuilder insert(int offset, float f)
public AbstractStringBuilder insert(int offset, double d)

在append和insert等函数的时候，需要判断进行数组扩容，expandCapacity函数完成数组的扩容，具体处理逻辑如下：

void expandCapacity(int minimumCapacity) {
    //原来数组的两倍+2(为什么这样设置，而不是2倍或其他)
    int newCapacity = value.length * 2 + 2;
    if (newCapacity - minimumCapacity < 0)
    //如果newCapacity小于minimumCapacity，则将扩容大小设置为minimumCapacity
        newCapacity = minimumCapacity;
    if (newCapacity < 0) {
        if (minimumCapacity < 0) // overflow
            throw new OutOfMemoryError();
        newCapacity = Integer.MAX_VALUE;
    }
    //调用Arrays.copyOf将value复制到扩容后的数组
    value = Arrays.copyOf(value, newCapacity);
}

public void ensureCapacity(int minimumCapacity) //子类可以调用的方法实现扩容，在方法中调用ensureCapacityInternal
private void ensureCapacityInternal(int minimumCapacity) //ensureCapacityInternal调用ensureCapacityInternal

其他方法

public int length() 返回字符串长度

public int capacity() 返回数组大小

public AbstractStringBuilder appendCodePoint(int codePoint)

public charAt(int index) 返回字符串中第（index+1）个字符

public AbstractStringBuilder deleteCharAt(int index)

public void setLength(int newLength) 设置字符串长度为newLength，如果newLength长度大于count，则以newLength为字符串长度，同时[count,newLengt-1]区间设置字符为'/0';当newLength长度小于count，则字符串value[0,newLength-1]

public void trimToSize() 如果字符串小于数组长度，将数组长度设置为字符串

public int indexOf(String str) //返回str在字符串中的位置

public int lastIndexOf(String str) //返回str在字符串中最后的位置

public AbstractStringBuilder reverse()//反转

final char[] getValue() 返回数组

public String substring(int start, int end)

public CharSequence subSequence(int start, int end)

public AbstractStringBuilder replace(int start, int end, String str)

StringBuilder

public final class StringBuilder extends AbstractStringBuilder implements java.io.Serializable, CharSequence

构造函数
public StringBuilder() {
super(16);//默认创建数组大小为16
}
public StringBuilder(int capacity)

public StringBuilder(String str) {
    super(str.length() + 16);
    append(str);
}
public StringBuilder(CharSequence seq) {
    this(seq.length() + 16);
    append(seq);
}

其他方法覆盖了AbstractStringBuilder中的方法，然后在方法体内调用对应的AbstractStringBuilder方法。

StringBuffer

StringBuffer与StringBuilder类似，只是方法用synchronized进行了修饰。