StringBuffer类之详解（二）

StringBuffer类之详解（一）

六、替换功能

StringBuffer

replace(int start, int end, String str)           Replaces the characters in a substring of this sequence with characters in the specified String.

从start开始到end用str替换

 @Override
    public synchronized StringBuffer replace(int start, int end, String str) {
        toStringCache = null;
        super.replace(start, end, str);
        return this;
    }

public AbstractStringBuilder replace(int start, int end, String str) {
        //越界异常
        if (start < 0)
            throw new StringIndexOutOfBoundsException(start);
        if (start > count)
            throw new StringIndexOutOfBoundsException("start > length()");
        if (start > end)
            throw new StringIndexOutOfBoundsException("start > end");
        //end大于缓冲区长度，则替换到最后一位
        if (end > count)
            end = count;
        int len = str.length();
        //包含头，不包含尾的替换
        int newCount = count + len - (end - start);
        ensureCapacityInternal(newCount);
        //缓冲区字符数组从end开始将count-end长度复制到该数组start+len处开始
        System.arraycopy(value, end, value, start + len, count - end);
        //start开始插入str字符串
        str.getChars(value, start);
        count = newCount;
        return this;
    }

七、反转功能

StringBuffer

reverse()           Causes this character sequence to be replaced by the reverse of the sequence.

字符串反转

@Override
    public synchronized StringBuffer reverse() {
        toStringCache = null;
        super.reverse();
        return this;
    }

public AbstractStringBuilder reverse() {
        boolean hasSurrogates = false;
        //n为缓冲区数组下标
        int n = count - 1;
        //j相当于(n-1)/2
        for (int j = (n-1) >> 1; j >= 0; j--) {
            //交换对应的两位字符
            int k = n - j;
            char cj = value[j];
            char ck = value[k];
            value[j] = ck;
            value[k] = cj;
            if (Character.isSurrogate(cj) ||
                Character.isSurrogate(ck)) {
                hasSurrogates = true;
            }
        }
        if (hasSurrogates) {
            reverseAllValidSurrogatePairs();
        }
        return this;
    }

八、截取功能

String	substring(int start)           Returns a new String that contains a subsequence of characters currently contained in this character sequence.
String	substring(int start, int end)           Returns a new String that contains a subsequence of characters currently contained in this sequence.

从指定位置截取到末尾

截取从指定位置开始到结束位置，包括开始位置，不包括结束位置

 @Override
    public synchronized String substring(int start) {
        return substring(start, count);
    }

@Override
    public synchronized String substring(int start, int end) {
        return super.substring(start, end);
    }

public String substring(int start, int end) {
        if (start < 0)
            throw new StringIndexOutOfBoundsException(start);
        if (end > count)
            throw new StringIndexOutOfBoundsException(end);
        if (start > end)
            throw new StringIndexOutOfBoundsException(end - start);
        return new String(value, start, end - start);
    }

 注：返回类型不在是StringBuffer对象，而是字符串对象，如需使用，要及时接受返回数据。

九、String转化为StringBuffer

方式一：

通过StringBuffer构造方法

StringBuffer sb = new StringBuffer("123");

方式二：

通过StringBuffer的append()方法

StringBuffer sb2 = new StringBuffer();
sb2.append("123");					//通过append方法将字符串转换为StringBuffer对象

 十、StringBuffer转化为String

方式一：

通过String构造方法

StringBuffer sb = new StringBuffer("123");
String s1 = new String(sb);					

方式二：

通过StringBuffer的toString()方法

StringBuffer sb = new StringBuffer("123");
String s2 = sb.toString();

方式三：

通过StringBuffer的subString(0,length);

StringBuffer sb = new StringBuffer("123");
String s3 = sb.substring(0, sb.length());

十一、String、StringBuffer作为参数传递

基本数据类型作为参数时，进行的是传值，不改变原内容

引用数据类型作为参数时，进行的是传址，改变原内容

String虽然是引用数据类型，但是作为参数时他的传递是和基本数据类型一样