源码阅读笔记1 --- String

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    //此处final表示String类不可以被继承
public final class String
    implements java.io.Serializable, Comparable<String>, CharSequence {
    //value这个字符数组被用于字符储存，用private修饰并且没有set方法（没有提供可变的接口），说明数组不可变，数组里的内容也不可以变
    private final char value[];

    public String() {
        this.value = "".value;
    }

    public String(String original) {
        this.value = original.value;
        this.hash = original.hash;
    }

    //也可以传字符数组，将传进来的字符数组copy到value数组中
    public String(char value[]) {
        this.value = Arrays.copyOf(value, value.length);
    }

    public String(char value[], int offset, int count) {
        if (offset < 0) {
            throw new StringIndexOutOfBoundsException(offset);
        }
        if (count <= 0) {
            if (count < 0) {
                throw new StringIndexOutOfBoundsException(count);
            }
            if (offset <= value.length) {
                this.value = "".value;
                return;
            }
        }
        // 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);
    }

    //返回value数组中下标为index的字符，若是越界的话就抛出一个异常
    public char charAt(int index) {
        if ((index < 0) || (index >= value.length)) {
            throw new StringIndexOutOfBoundsException(index);
        }
        return value[index];
    }

    //String类重写Object类中的equal方法，父类引用指向子类对象
    public boolean equals(Object anObject) {
        if (this == anObject) {
            return true;
        }
        //instanceof 运算符是用来在运行时指出对象是否是特定类的一个实例
        if (anObject instanceof String) {
            String anotherString = (String)anObject;
            int n = value.length;
            if (n == anotherString.value.length) {
                char v1[] = value;
                char v2[] = anotherString.value;
                int i = 0;
                while (n-- != 0) {
                    if (v1[i] != v2[i])
                        return false;
                    i++;
                }
                return true;
            }
        }
        return false;
    }

    //忽略大小写
    public boolean equalsIgnoreCase(String anotherString) {
        return (this == anotherString) ? true
                : (anotherString != null)
                && (anotherString.value.length == value.length)
                && regionMatches(true, 0, anotherString, 0, value.length);
    }

    //返回字符ch对应的下标
    public int indexOf(int ch) {
        return indexOf(ch, 0);
    }

    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 trim() {
        int len = value.length;
        int st = 0;
        char[] val = value;    /* avoid getfield opcode */

        while ((st < len) && (val[st] <= ' ')) {
            st++;
        }
        while ((st < len) && (val[len - 1] <= ' ')) {
            len--;
        }
        return ((st > 0) || (len < value.length)) ? substring(st, len) : this;
    }
}