String 源码-优快云博客

博客介绍Python相关内容，重点阐述了String类。String不能被继承，实现多个接口，成员变量value[]为final，定义后不可变。还列举了getBytes、equals等常用方法，如getBytes交给StringCoding.encode获取bytes，regionMatches对传入参数合法性判断后比较字符。

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一、签名：

public final class String implements java.io.Serializable, Comparable<String>, CharSequence

String不能被继承，实现了Serializable：可被序列化。

实现了Comparable：可以比较，排序，实现了CharSequence：值是可读序列。

二、成员变量：

private static final long serialVersionUID = -6849794470754667710L;
private final char value[];// 存储字符。
private int hash;// 缓存字符串的hash值。
Private static final ObjectStreamField[] serialPersistentFields = new ObjectStreamFiled[0];  // 字符串类被指定转化序列化流协议中

value[]是final的，所以String一旦定义之后，就不可变的。

三、常用方法：

1.getBytes 将字符串转为字节数组

public byte[] getBytes() {
   return StringCoding.encode(value, 0, value.length);
}

以上面的这个方法为例：它交给了StringCoding.encode去获取bytes。然而StringEncoder去调用了StringEncoder的相应方法获取bytes

2.equals 比较两个字符串是否相等

public boolean equals(Object anObject) {
        if (this == anObject) {   // 如果地址都相同的话，那肯定就是同一个了
            return true;
        }

        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;
    }

3.equalsIgnoreCase 比较忽略大小写之后两个字符串是否相等

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

}

public boolean regionMatches(boolean ignoreCase, int toffset,String other, int ooffset, int len) {
        char ta[] = value;
        int to = toffset;
        char pa[] = other.value;
        int po = ooffset;
        if ((ooffset < 0) || (toffset < 0)
                || (toffset > (long)value.length - len)
                || (ooffset > (long)other.value.length - len)) {
            return false;
        }

        while (len-- > 0) {
            char c1 = ta[to++];
            char c2 = pa[po++];
            if (c1 == c2) {
                continue;
            }
            if (ignoreCase) {
                char u1 = Character.toUpperCase(c1);
                char u2 = Character.toUpperCase(c2);
                if (u1 == u2) {
                    continue;
                }
                if (Character.toLowerCase(u1) == Character.toLowerCase(u2)) {
                    continue;
                }
            }
            return false;
        }
        return true;
}

在regionMatches中首先对传入的参数，进行合法性判断。然后取出每一个字符，比较一下，相同就下个字符，不同，都转换成大写，比较一次，如果相同就比较下一个字符，如果不相同就都转化成小写在比较一下。解释是说：有时候，转大写不一定每次都会正确。

4.startsWith 判断字符串是否从toffset位置开始以prefix开头

public boolean startsWith(String prefix, int toffset) {
        char ta[] = value;
        int to = toffset;
        char pa[] = prefix.value;
        int po = 0;
        int pc = prefix.value.length;
        if ((toffset < 0) || (toffset > value.length - pc)) {
            return false;
        }
        while (--pc >= 0) {
            if (ta[to++] != pa[po++]) {
                return false;
            }
        }
        return true;
}

5.endsWith 判断字符串是否以suffix结尾

public boolean endsWith(String suffix) {
     return startsWith(suffix, value.length - suffix.value.length);
}

6.hashCode 返回字符串对应的hash码

public int hashCode() {
        int h = hash;  ///先获取缓存中的hash值
        if (h == 0 && value.length > 0) {//若为默认值0,则进行计算
            char val[] = value;
            for (int i = 0; i < value.length; i++) {
                h = 31 * h + val[i];//hash值算法 s[0]*31^(n-1) + s[1]*31^(n-2) + ... + s[n-1]
            }
            hash = h;
        }
        return h;
}

7.replace 用newchar去替换所有的oldChar

public String replace(char oldChar, char newChar) {
        if (oldChar != newChar) {//若oldChar和newChar相同,则直接返回
            int len = value.length;
            int i = -1;
            char[] val = value;
            while (++i < len) {
                if (val[i] == oldChar) {
                    break;
                }
            }
            if (i < len) {
                char buf[] = new char[len];
                for (int j = 0; j < i; j++) {
                    buf[j] = val[j];
                }
                while (i < len) {
                    char c = val[i];
                    buf[i] = (c == oldChar) ? newChar : c;
                    i++;
                }
                return new String(buf, true);
            }
        }
        return this;
}

8.length() String中字符的个数

public int length() {
        return value.length;
    }

9.charAt() 取得索引位置上的字符

public char charAt(int index) {
    if ((index < 0) || (index >= value.length)) {
        throw new StringIndexOutOfBoundsException(index);
    }
    return value[index];
}

转载于:https://my.oschina.net/langwanghuangshifu/blog/1934503