Java基礎系列-substring的原理

程序

public static void main(String[] args) {
    String s = "字符串截取";
    String substring = s.substring(0,2);
    System.out.println(substring); // 字符
}

public String substring(int beginIndex, int endIndex) {
    if (beginIndex < 0) {
        throw new StringIndexOutOfBoundsException(beginIndex);
    }
    if (endIndex > value.length) {
        throw new StringIndexOutOfBoundsException(endIndex);
    }
    int subLen = endIndex - beginIndex;
    if (subLen < 0) {
        throw new StringIndexOutOfBoundsException(subLen);
    }
    return ((beginIndex == 0) && (endIndex == value.length)) ? this
            : new String(value, beginIndex, subLen);
}

public String(char value[], int offset, int count) {
    if (offset < 0) {
        throw new StringIndexOutOfBoundsException(offset);
    }
    if (count < 0) {
        throw new StringIndexOutOfBoundsException(count);
    }
    // Note: offset or count might be near -1>>>1.
    if (offset > value.length - count) {
        throw new StringIndexOutOfBoundsException(offset + count);
    }
    this.value = Arrays.copyOfRange(value, offset, offset+count);
}

public static char[] copyOfRange(char[] original, int from, int to) {
    int newLength = to - from;
    if (newLength < 0)
        throw new IllegalArgumentException(from + " > " + to);
    char[] copy = new char[newLength];
    System.arraycopy(original, from, copy, 0,
                     Math.min(original.length - from, newLength));
    return copy;
}

本地方法

public static native void arraycopy(Object src,  int  srcPos,
                                    Object dest, int destPos,
                                    int length);

說明：

substring方法返回的字符串對象時新建立的，可是底層指向的字符數組並未改變，還指向原字符串對象指向的字符數組，僅僅是使用對象中的三個屬性來限制字符數組的開始位置與長度，展示給咱們的就是截取完的新字符串。

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