String 源码

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java.lang.Stringfinal class String    implements java.io.Serializable, Comparable<String>, CharSequence

1.类变量&常量

    private final char value[];     //用于存储值的char型数组

    private int hash; // 默认为0

    //新建一个CaseInsensitiveComparator类，用于调用其compareTo方法    public static final Comparator<String> CASE_INSENSITIVE_ORDER                                         = new CaseInsensitiveComparator();

2.构造函数

    //默认构造方式    public String() {                   this.value = "".value;    }    //下面是对byte[]数组的编码的构造方式    public String(byte bytes[]) {        this(bytes, 0, bytes.length);    }    public String(byte bytes[], String charsetName)            throws UnsupportedEncodingException {        this(bytes, 0, bytes.length, charsetName);    }    public String(byte bytes[], Charset charset) {        this(bytes, 0, bytes.length, charset);    }    public String(byte bytes[], int offset, int length) {        checkBounds(bytes, offset, length);     //检查边界条件        //以默认的编码方式进行编码，若无则以ISO-8859-1编码        this.value = StringCoding.decode(bytes, offset, length);    }    //以charsetName的编码方式对bytes[]数组进行编码    public String(byte bytes[], int offset, int length, String charsetName)            throws UnsupportedEncodingException {        if (charsetName == null)            throw new NullPointerException("charsetName");        checkBounds(bytes, offset, length);        this.value = StringCoding.decode(charsetName, bytes, offset, length);    }    public String(byte bytes[], int offset, int length, Charset charset) {        if (charset == null)            throw new NullPointerException("charset");        checkBounds(bytes, offset, length);        this.value =  StringCoding.decode(charset, bytes, offset, length);    }    //下面是输入char[]数组的构造方式    public String(char value[]) {        this.value = Arrays.copyOf(value, value.length);    //对value[]进行拷贝    }    //为了更快的对String进行构造    String(char[] value, boolean share) {        // assert share : "unshared not supported";        this.value = value;    }    //有边界范围的拷贝    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);    }    //下面是以String参数的构造方法    public String(String original) {        this.value = original.value;        this.hash = original.hash;    }    //同步的构造方法，以StringBuffer作为参数    public String(StringBuffer buffer) {        synchronized(buffer) {            this.value = Arrays.copyOf(buffer.getValue(), buffer.length());        }    }    //非同步的构造方法，以StringBuilder作为参数    public String(StringBuilder builder) {        this.value = Arrays.copyOf(builder.getValue(), builder.length());    }

3.内部类

CaseInsensitiveComparator implements Comparator<String>, java.io.Serializable 继承了Comparator接口，实现了compareTo方法，对String类型的数进行比较，忽略大小写    public int compare(String s1, String s2) {            int n1 = s1.length();            int n2 = s2.length();            int min = Math.min(n1, n2);     //只比较到二者最小的长度            for (int i = 0; i < min; i++) {                char c1 = s1.charAt(i);                char c2 = s2.charAt(i);                if (c1 != c2) {                    c1 = Character.toUpperCase(c1);                    c2 = Character.toUpperCase(c2);                    if (c1 != c2) {                        c1 = Character.toLowerCase(c1);                        c2 = Character.toLowerCase(c2);                        if (c1 != c2) {                            // 不会发生逸出，以为char类型为16位                            return c1 - c2;                        }                    }                }            }            return n1 - n2;        }

4.重要方法

1.indexOf方法，返回某个char或者String子串在源字符串中首次出现的位置、    public int indexOf(int ch) {        return indexOf(ch, 0);    }    public int indexOf(int ch, int fromIndex) { //注意这里的输入参数类型是int        final int max = value.length;        if (fromIndex < 0) {            fromIndex = 0;        } else if (fromIndex >= max) {            // Note: fromIndex might be near -1>>>1.            return -1;        }        if (ch < Character.MIN_SUPPLEMENTARY_CODE_POINT) {            // handle most cases here (ch is a BMP code point or a            // negative value (invalid code point))            final char[] value = this.value;            for (int i = fromIndex; i < max; i++) {                if (value[i] == ch) {                    return i;                }            }            return -1;        } else {            return indexOfSupplementary(ch, fromIndex);        }    }    public int indexOf(String str) {        return indexOf(str, 0);    }    public int indexOf(String str, int fromIndex) {        return indexOf(value, 0, value.length,                str.value, 0, str.value.length, fromIndex);    }    static int indexOf(char[] source, int sourceOffset, int sourceCount,            String target, int fromIndex) {        return indexOf(source, sourceOffset, sourceCount,                       target.value, 0, target.value.length,                       fromIndex);    }    //查找指定子串的位置    static int indexOf(char[] source, int sourceOffset, int sourceCount,            char[] target, int targetOffset, int targetCount,            int fromIndex) {        if (fromIndex >= sourceCount) {            return (targetCount == 0 ? sourceCount : -1);        }        if (fromIndex < 0) {            fromIndex = 0;        }        if (targetCount == 0) {            return fromIndex;        }        char first = target[targetOffset];        int max = sourceOffset + (sourceCount - targetCount);        for (int i = sourceOffset + fromIndex; i <= max; i++) {            /* Look for first character. */            if (source[i] != first) {                while (++i <= max && source[i] != first);//一直循环直到找到第一个字符            }            /* Found first character, now look at the rest of v2 */            if (i <= max) {                int j = i + 1;                int end = j + targetCount - 1;                for (int k = targetOffset + 1; j < end && source[j]                        == target[k]; j++, k++);                if (j == end) {                    /* Found whole string. */                    return i - sourceOffset;    //返回第一个字符串的位置                }            }        }        return -1;    }

2.valueOf函数，静态且返回类型为一个新的String字符串    public static String valueOf(boolean b) {        return b ? "true" : "false";    }    public static String valueOf(char c) {        char data[] = {c};        return new String(data, true);    }    public static String valueOf(int i) {        return Integer.toString(i);    }    public static String valueOf(long l) {        return Long.toString(l);    }    public static String valueOf(float f) {        return Float.toString(f);    }    public static String valueOf(double d) {        return Double.toString(d);    }

3.返回指定的位置上的charpublic char charAt(int index) {        if ((index < 0) || (index >= value.length)) {            throw new StringIndexOutOfBoundsException(index);        }        return value[index];    }

4.compareTo，继承自Comparable接口，比较两个字符串的大小    public int compareTo(String anotherString) {        int len1 = value.length;        int len2 = anotherString.value.length;        int lim = Math.min(len1, len2);        char v1[] = value;        char v2[] = anotherString.value;        int k = 0;        while (k < lim) {            char c1 = v1[k];            char c2 = v2[k];            if (c1 != c2) { //只要有不相同的字符出现，立即比较并返回                return c1 - c2;            }            k++;        }        return len1 - len2;    }

5.连接两个字符串concat    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);        return new String(buf, true);    }    void getChars(char dst[],int dstBegin){//将value的值拷贝到dst数组(dstBegin处开始)        System.arraycopy(value, 0, dst, dstBegin, value.length);    }

6.求hash值&equals方法&toString方法，继承自Object类    public int hashCode() {        int h = hash;        if (h == 0 && value.length > 0) {            char val[] = value;            for (int i = 0; i < value.length; i++) {                h = 31 * h + val[i];        //求hash值得表达式            }            hash = h;        }        return h;    }    public boolean equals(Object anObject) {        if (this == anObject) {            return true;        }        if (anObject instanceof String) {            String anotherString = (String)anObject;            int n = value.length;            if (n == anotherString.value.length) {  //先比较二者长度                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 String toString() {        return this;    }

7.替换函数replace，将oldChar替换成newChar,完全返回一个new String    public String replace(char oldChar, char newChar) {        if (oldChar != newChar) {            int len = value.length;            int i = -1;            char[] val = value; /* avoid getfield opcode */            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.变换成char数组    public char[] toCharArray() {        //因为class文件的初始化顺序问题，不能用Arrays.copyOf方法        char result[] = new char[value.length];        System.arraycopy(value, 0, result, 0, value.length);        return result;    }

9.trim方法，返回一个新的字符串，其首部和尾部均不能为空格，其它和源字符串无区别    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;    }

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