java String部分源码学习记录

Posted 2020-06-09

public final class String
    implements java.io.Serializable, Comparable<String>, CharSequence {
    /**char数组用于字符的存储 */
    private final char value[];

    /** 缓存string的hash码 */
    private int hash; // Default to 0

public String() {/**无参构造函数,打印值为""*/
        this.value = new char[0];
    }

public String(String original) {/**字符值和hash值均为参数的值*/
this.value = original.value;
        this.hash = original.hash;
    }

 public String(char value[]) {/**Arrays.copyOf()方法返回一个新构造的
char数组*/
        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) {
            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);
    }

/**传入代码点来构造String对象,代码点和代码单元相关内容可自行查阅资料,或者看我另一篇随笔深入学习Java中的字符串,代码点和代码单元*/

public String(int[] codePoints, 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 > codePoints.length - count) {
            throw new StringIndexOutOfBoundsException(offset + count);
        }

        final int end = offset + count;

        /**计算char数组的真实大小,一个代码点有可能需要用2个char即2个代码单元表示*/
        int n = count;
        for (int i = offset; i < end; i++) {
            int c = codePoints[i];
            if (Character.isBmpCodePoint(c))/**此代码点为基本字符代码点*/
                continue;
            else if (Character.isValidCodePoint(c))/**此代码点为有效代码点,即辅助字符代码点,此时一个代码点需要用2个char即2个代码单元表示,n++*/
                n++;
            else throw new IllegalArgumentException(Integer.toString(c));
        }

        /**分配并填充字符数组*/
        final char[] v = new char[n];

        for (int i = offset, j = 0; i < end; i++, j++) {
            int c = codePoints[i];
            if (Character.isBmpCodePoint(c))
                v[j] = (char)c;
            else
                Character.toSurrogates(c, v, j++);
        }

        this.value = v;
    }

/**返回指定下标的字符*/
 public char charAt(int index) {
        if ((index < 0) || (index >= value.length)) {
            throw new StringIndexOutOfBoundsException(index);
        }
        return value[index];
    }

/**返回指定下标的代码点*/
public int codePointAt(int index) {
        if ((index < 0) || (index >= value.length)) {
            throw new StringIndexOutOfBoundsException(index);
        }
        return Character.codePointAtImpl(value, index, value.length);
    }

public boolean equals(Object anObject) {
        if (this == anObject) {
            return true;/**如果是同一个对象,直接返回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;
    }

public boolean contentEquals(StringBuffer sb) {
        synchronized (sb) {/**锁定StringBuffer对象,防止修改*/
            return contentEquals((CharSequence) sb);
        }
    }

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

/**计算并返回hsah码*/
public int hashCode() {
        int h = hash;/**默认是0*/
        if (h == 0 && value.length > 0) {
            char val[] = value;

            for (int i = 0; i < value.length; i++) {
                h = 31 * h + val[i];
            }
            hash = h;
        }
        return h;
    }

/**字串为新对象*/
public String substring(int beginIndex) {
        if (beginIndex < 0) {
            throw new StringIndexOutOfBoundsException(beginIndex);
        }
        int subLen = value.length - beginIndex;
        if (subLen < 0) {
            throw new StringIndexOutOfBoundsException(subLen);
        }
        return (beginIndex == 0) ? this : new String(value, beginIndex, subLen);
    }
/**连接字符串*/
public String concat(String str) {
        int otherLen = str.length();
        if (otherLen == 0) {
            return this;
        }
        int len = value.length;
        char buf[] = Arrays.copyOf(value, len + otherLen);
        str.getChars(buf, len);/**将str内的字符依次存放到buf数组中,下标从len开始*/
        return new String(buf, true);/**返回新生成的对象*/
    }

/**trim()方法将字符串首尾的ascii码数值小于或等于空格的字符删除*/
public String trim() {
        int len = value.length;
        int st = 0;
        char[] val = value;    /* avoid getfield opcode */

        while ((st < len) && (val[st] <= ‘ ‘)) {/**从字符串首个字符开始判断ascii是否小于或等于空格,若小于或等于,计数++*/
            st++;
        }
        while ((st < len) && (val[len - 1] <= ‘ ‘)) {/**从字符串最后一个字符开始判断ascii是否小于或等于空格,若小于或等于,计数--*
            len--;
        }
        return ((st > 0) || (len < value.length)) ? substring(st, len) : this;
    }

/**返回自身对象的引用*/
public String toString() {
        return this;
    }

/**和toString()唯一区别就是先判断对象是否为空*/
public static String valueOf(Object obj) {
        return (obj == null) ? "null" : obj.toString();
    }