数据结构---Java---String、StringBuilder、StringBuffer

1、概述

　　　　　　1.1　　String：不可变字符串

　　　　　　　　String的存储位置：

        String s1="a";
        String s2="a";
        String s3=new String("a");
        System.out.println(s1==s2);//true
        System.out.println(s1==s3);//false

　　　　　　String s="a";

　　　　　　　　一个对象，常量池“a”

　　　　　　　　这种方式创建字符串，首先去方法区的常量池找是否有 "a"对象，如果有，直接返回；没有在常量池创建一个"a"对象；

　　　　　　String s3=new String("a");

　　　　　　　　创建2个对象，一个在常量池“a”、一个在堆s3指向的字符串对象；

　　　　　　　　首先去常量池找是否有"a"字符串，如果没有，常量池创建"a"对象，堆创建 s3指向的对象；

　　　　　　1.2、StringBuffer：可改变的字符串，线程安全

abstract class AbstractStringBuilder implements Appendable, CharSequence {

    char[] value;

    AbstractStringBuilder(int capacity) {
        value = new char[capacity];
    }
    
    public AbstractStringBuilder append(String str) {
        if (str == null)
            return appendNull();
        int len = str.length();
        ensureCapacityInternal(count + len);
        str.getChars(0, len, value, count);
        count += len;
        return this;
    }

    public AbstractStringBuilder append(StringBuffer sb) {
        if (sb == null)
            return appendNull();
        int len = sb.length();
        ensureCapacityInternal(count + len);
        sb.getChars(0, len, value, count);
        count += len;
        return this;
    }


    public AbstractStringBuilder reverse() {
        boolean hasSurrogates = false;
        int n = count - 1;
        for (int j = (n-1) >> 1; j >= 0; j--) {
            int k = n - j;
            char cj = value[j];
            char ck = value[k];
            value[j] = ck;
            value[k] = cj;
            if (Character.isSurrogate(cj) ||
                Character.isSurrogate(ck)) {
                hasSurrogates = true;
            }
        }
        if (hasSurrogates) {
            reverseAllValidSurrogatePairs();
        }
        return this;
    }

}



 public final class StringBuffer
    extends AbstractStringBuilder
    implements java.io.Serializable, CharSequence
{

    public StringBuffer() {
        super(16);
    }


    @Override
    public synchronized StringBuffer append(String str) {
        toStringCache = null;
        super.append(str);
        return this;
    }


    public synchronized StringBuffer append(StringBuffer sb) {
        toStringCache = null;
        super.append(sb);
        return this;
    }


    @Override
    public synchronized StringBuffer reverse() {
        toStringCache = null;
        super.reverse();
        return this;
    }

}

　　　　　　1.3、 StringBuilder：可改变的字符串，线程不安全

abstract class AbstractStringBuilder implements Appendable, CharSequence {

    char[] value;

    AbstractStringBuilder(int capacity) {
        value = new char[capacity];
    }
    
    public AbstractStringBuilder append(String str) {
        if (str == null)
            return appendNull();
        int len = str.length();
        ensureCapacityInternal(count + len);
        str.getChars(0, len, value, count);
        count += len;
        return this;
    }

    public AbstractStringBuilder append(StringBuffer sb) {
        if (sb == null)
            return appendNull();
        int len = sb.length();
        ensureCapacityInternal(count + len);
        sb.getChars(0, len, value, count);
        count += len;
        return this;
    }


    public AbstractStringBuilder reverse() {
        boolean hasSurrogates = false;
        int n = count - 1;
        for (int j = (n-1) >> 1; j >= 0; j--) {
            int k = n - j;
            char cj = value[j];
            char ck = value[k];
            value[j] = ck;
            value[k] = cj;
            if (Character.isSurrogate(cj) ||
                Character.isSurrogate(ck)) {
                hasSurrogates = true;
            }
        }
        if (hasSurrogates) {
            reverseAllValidSurrogatePairs();
        }
        return this;
    }

}


public final class StringBuilder
    extends AbstractStringBuilder
    implements java.io.Serializable, CharSequence
{

    public StringBuilder() {
        super(16);
    }

    public StringBuilder(int capacity) {
        super(capacity);
    }

    @Override
    public StringBuilder append(String str) {
        super.append(str);
        return this;
    }

    public StringBuilder append(StringBuffer sb) {
        super.append(sb);
        return this;
    }

    @Override
    public StringBuilder reverse() {
        super.reverse();
        return this;
    }

     @Override
    public String toString() {
        // Create a copy, don't share the array
        return new String(value, 0, count);
    }

}

2、字符串操作

　　　　2.1、返回指定位置的字符

String

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


StringBuilder

无


StringBuffer

@Override
    public synchronized char charAt(int index) {
        if ((index < 0) || (index >= count))
            throw new StringIndexOutOfBoundsException(index);
        return value[index];
    }

　　　　2.2、返回指定字符的位置

1、String

public int indexOf(int ch) {
        return indexOf(ch, 0);
    }

public int indexOf(int ch, int fromIndex) {
        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);
        }
    }


2、StringBuilder

@Override
    public int indexOf(String str) {
        return super.indexOf(str);
    }

使用父类AbstractStringBuilder

public int indexOf(String str) {
        return indexOf(str, 0);
    }

public int indexOf(String str, int fromIndex) {
        return String.indexOf(value, 0, count, str, fromIndex);
    }

使用String

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);
    }


3、StringBuffer

@Override
    public int indexOf(String str) {
        // Note, synchronization achieved via invocations of other StringBuffer methods
        return super.indexOf(str);
    }

同 StringBuilder

　　　　2.3、字符串截取

1、String

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);
    }


2、StringBuilder

无


3、StringBuffer

@Override
    public synchronized String substring(int start) {
        return substring(start, count);
    }

@Override
    public synchronized String substring(int start, int end) {
        return super.substring(start, end);
    }

使用父类 AbstractStringBuilder
public String substring(int start, int end) {
        if (start < 0)
            throw new StringIndexOutOfBoundsException(start);
        if (end > count)
            throw new StringIndexOutOfBoundsException(end);
        if (start > end)
            throw new StringIndexOutOfBoundsException(end - start);
        return new String(value, start, end - start);
    }

　　　　2.4、字符串拼接

1、String

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);
    }


2、StringBuilder

@Override
    public StringBuilder append(String str) {
        super.append(str);
        return this;
    }


父类AbstractStringBuilder

public AbstractStringBuilder append(String str) {
        if (str == null)
            return appendNull();
        int len = str.length();
        ensureCapacityInternal(count + len);
        str.getChars(0, len, value, count);
        count += len;
        return this;
    }

3、StringBuffer

@Override
    public synchronized StringBuffer append(String str) {
        toStringCache = null;
        super.append(str);
        return this;
    }

父类AbstractStringBuilder

public AbstractStringBuilder append(String str) {
        if (str == null)
            return appendNull();
        int len = str.length();
        ensureCapacityInternal(count + len);
        str.getChars(0, len, value, count);
        count += len;
        return this;
    }

　　　　2.5、按指定规则切割

1、String

public String[] split(String regex) {
        return split(regex, 0);
    }

public String[] split(String regex, int limit) {
        /* fastpath if the regex is a
         (1)one-char String and this character is not one of the
            RegEx's meta characters ".$|()[{^?*+\\", or
         (2)two-char String and the first char is the backslash and
            the second is not the ascii digit or ascii letter.
         */
        char ch = 0;
        if (((regex.value.length == 1 &&
             ".$|()[{^?*+\\".indexOf(ch = regex.charAt(0)) == -1) ||
             (regex.length() == 2 &&
              regex.charAt(0) == '\\' &&
              (((ch = regex.charAt(1))-'0')|('9'-ch)) < 0 &&
              ((ch-'a')|('z'-ch)) < 0 &&
              ((ch-'A')|('Z'-ch)) < 0)) &&
            (ch < Character.MIN_HIGH_SURROGATE ||
             ch > Character.MAX_LOW_SURROGATE))
        {
            int off = 0;
            int next = 0;
            boolean limited = limit > 0;
            ArrayList<String> list = new ArrayList<>();
            while ((next = indexOf(ch, off)) != -1) {
                if (!limited || list.size() < limit - 1) {
                    list.add(substring(off, next));
                    off = next + 1;
                } else {    // last one
                    //assert (list.size() == limit - 1);
                    list.add(substring(off, value.length));
                    off = value.length;
                    break;
                }
            }
            // If no match was found, return this
            if (off == 0)
                return new String[]{this};

            // Add remaining segment
            if (!limited || list.size() < limit)
                list.add(substring(off, value.length));

            // Construct result
            int resultSize = list.size();
            if (limit == 0) {
                while (resultSize > 0 && list.get(resultSize - 1).length() == 0) {
                    resultSize--;
                }
            }
            String[] result = new String[resultSize];
            return list.subList(0, resultSize).toArray(result);
        }
        return Pattern.compile(regex).split(this, limit);
    }


2、StringBuilder

无

3、StringBuffer

无

　　　　2.6、字符串倒序

1、String

无


2、StringBuilder

@Override
    public StringBuilder reverse() {
        super.reverse();
        return this;
    }

父类AbstractStringBuilder

public AbstractStringBuilder reverse() {
        boolean hasSurrogates = false;
        int n = count - 1;
        for (int j = (n-1) >> 1; j >= 0; j--) {
            int k = n - j;
            char cj = value[j];
            char ck = value[k];
            value[j] = ck;
            value[k] = cj;
            if (Character.isSurrogate(cj) ||
                Character.isSurrogate(ck)) {
                hasSurrogates = true;
            }
        }
        if (hasSurrogates) {
            reverseAllValidSurrogatePairs();
        }
        return this;
    }

3、StringBuffer

@Override
    public synchronized StringBuffer reverse() {
        toStringCache = null;
        super.reverse();
        return this;
    }

父类AbstractStringBuilder

public AbstractStringBuilder reverse() {
        boolean hasSurrogates = false;
        int n = count - 1;
        for (int j = (n-1) >> 1; j >= 0; j--) {
            int k = n - j;
            char cj = value[j];
            char ck = value[k];
            value[j] = ck;
            value[k] = cj;
            if (Character.isSurrogate(cj) ||
                Character.isSurrogate(ck)) {
                hasSurrogates = true;
            }
        }
        if (hasSurrogates) {
            reverseAllValidSurrogatePairs();
        }
        return this;
    }

3、字符串创建形式

　　　　3.1、String s ="1";

　　　　　　　　创建一个"1"对象，存放于常量池

　　　　3.2、String s2=new String("1");

　　　　　　　　创建一个 "1"对象或 2个 s2指向的对象+"1"对象