JDK6和JDK7中String的substring()方法及其差异

// JDK6,包级私有构造,共享 value数组提升速度  
String(int offset, int count, char value[]) {  
    this.value = value;  
    this.offset = offset;  
    this.count = count;  
}  
  
  
public String substring(int beginIndex, int endIndex) {  
    // ... 检查边界的代码  
    // 如果范围和自己一模一样,则返回自身,否则用value字符数组构造一个新的对象  
    return ((beginIndex == 0) && (endIndex == count)) ? this :  
        new String(offset + beginIndex, endIndex - beginIndex, value);  
}

　　　　JDK6中value对象不变，只是offset和count 的两个属性值发生了变化 

 

    public String substring(int i, int j)
    {
        if(i < 0)
            throw new StringIndexOutOfBoundsException(i);
        if(j > value.length)
            throw new StringIndexOutOfBoundsException(j);
        int k = j - i;
        if(k < 0)
            throw new StringIndexOutOfBoundsException(k);
        else
            return i != 0 || j != value.length ? new String(value, i, k) : this;
    }


    public String(char ac[], int i, int j)
    {
        if(i < 0)
            throw new StringIndexOutOfBoundsException(i);
        if(j <= 0)
        {
            if(j < 0)
                throw new StringIndexOutOfBoundsException(j);
            if(i <= ac.length)
            {
                value = "".value;
                return;
            }
        }
        if(i > ac.length - j)
        {
            throw new StringIndexOutOfBoundsException(i + j);
        } else
        {
            value = Arrays.copyOfRange(ac, i, i + j);
            return;
        }
    }


    public String(char ac[], int i, int j)
    {
        if(i < 0)
            throw new StringIndexOutOfBoundsException(i);
        if(j <= 0)
        {
            if(j < 0)
                throw new StringIndexOutOfBoundsException(j);
            if(i <= ac.length)
            {
                value = "".value;
                return;
            }
        }
        if(i > ac.length - j)
        {
            throw new StringIndexOutOfBoundsException(i + j);
        } else
        {
            value = Arrays.copyOfRange(ac, i, i + j);
            return;
        }
    }

　　　　JDK7中真实地在堆内存中创建了另一个字符数组.