深入学习String

1.String的源码展示

public final class String
    implements java.io.Serializable, Comparable<String>, CharSequence
{
    /** The value is used for character storage. */
    private final char value[];

    /** The offset is the first index of the storage that is used. */
    private final int offset;

    /** The count is the number of characters in the String. */
    private final int count;

    /** Cache the hash code for the string */
    private int hash; // Default to 0

    /** use serialVersionUID from JDK 1.0.2 for interoperability */
    private static final long serialVersionUID = -6849794470754667710L;

    ........
}

String类是final类，不能被继承，而且其中成员方法也是final方法，可以看出String类中是通过char数组来保存数据的

2.下面看看其他方法的实现

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

public String concat(String str) {
    int otherLen = str.length();
    if (otherLen == 0) {
        return this;
    }
    char buf[] = new char[count + otherLen];
    getChars(0, count, buf, 0);
    str.getChars(0, otherLen, buf, count);
    return new String(0, count + otherLen, buf);
}

public String replace(char oldChar, char newChar) {
    if (oldChar != newChar) {
        int len = count;
        int i = -1;
        char[] val = value; /* avoid getfield opcode */
        int off = offset;   /* avoid getfield opcode */

        while (++i < len) {
        if (val[off + i] == oldChar) {
            break;
        }
        }
        if (i < len) {
        char buf[] = new char[len];
        for (int j = 0 ; j < i ; j++) {
            buf[j] = val[off+j];
        }
        while (i < len) {
            char c = val[off + i];
            buf[i] = (c == oldChar) ? newChar : c;
            i++;
        }
        return new String(0, len, buf);
        }
    }
    return this;
}

由以上方法可以看出这些方法都不是在原有的字符串上进行操作的，而是新生成了一个字符串对象，也就是说经过浙西操作之后原有的字符串并没有改变

在这里要永远记住一点：“String对象一旦被创建就是固定不变的了，对String对象的任何改变都不影响到原对象，相关的任何change操作都会生成新的对象”。