.java.lang.String

  由于1.8的rt.jar打开报错, 此番查看的是基于1.7

   java.lang.String 是一个final类.

 

   一, 接口

     实现三个接口: Serializable, Comparable<String>, CharSequence

 

   二, 属性

   

  private final char[] value;
  private int hash;
  private static final long serialVersionUID = -6849794470754667710L;
  private static final ObjectStreamField[] serialPersistentFields = new ObjectStreamField[0];
  public static final Comparator<String> CASE_INSENSITIVE_ORDER = new CaseInsensitiveComparator(null);
  private static final int HASHING_SEED = i;
  private transient int hash32 = 0;

     重要的属性有value和hash. 其中value是final,char数组存储的.

 

  三, 构造器

     java.lang.String的构造器比较多. 有个线程安全的构造器值得注意.

    

  public String(StringBuffer paramStringBuffer)
  {
    synchronized (paramStringBuffer)
    {
      this.value = Arrays.copyOf(paramStringBuffer.getValue(), paramStringBuffer.length());
    }
  }

 

  四,方法

    1, equals

          第一步:比较2个对象是否同一个对象,若是,直接返回true

          第二步: 判断比较类型是否是java.lang.String,若否, 直接false

          第三步:  判断字符长度是否相等, 若否, 直接false

          第四步: 从第一个字符开始进行比较,若发现不相等,直接返回false.

          细节: 当中判断用的是i++ !=0, 而不是我们平时j<this.value.length. 性能更佳.

      

  public boolean equals(Object paramObject)
  {
    if (this == paramObject)
      return true;
    if ((paramObject instanceof String))
    {
      String str = (String)paramObject;
      int i = this.value.length;
      if (i == str.value.length)
      {
        char[] arrayOfChar1 = this.value;
        char[] arrayOfChar2 = str.value;
        for (int j = 0; i-- != 0; j++)
          if (arrayOfChar1[j] != arrayOfChar2[j])
            return false;
        return true;
      }
    }
    return false;
  }

 

     2, equalsIgnoreCase

      直接比较地址是否相等,没有值比较.

  public boolean equalsIgnoreCase(String paramString)
  {
    return this == paramString;
  }

 

   3, contentEquals

     有2种, StringBuffer参数的调用CharSequence, 因为是线程安全的,所以加了synchronized.

      boolean contentEquals(StringBuffer paramStringBuffer)

      boolean contentEquals(CharSequence paramCharSequence)

 

    3, compareTo

      字符串集合进行排序的基础。 此方法比较2个字符串的大小。

      第一步： 从2个字符串中取出最小字符长度。

      第二步： 从0到最小字符串长度进行比较，若不相等，返回str1[i]-str[2]

      第三步:    其它, 返回str1.length -str2.length

  public int compareTo(String paramString)
  {
    int i = this.value.length;
    int j = paramString.value.length;
    int k = Math.min(i, j);
    char[] arrayOfChar1 = this.value;
    char[] arrayOfChar2 = paramString.value;
    for (int m = 0; m < k; m++)
    {
      int n = arrayOfChar1[m];
      int i1 = arrayOfChar2[m];
      if (n != i1)
        return n - i1;
    }
    return i - j;
  }

     4, startsWith

      判断字符串是否以指定字符串开头.

       boolean startsWith(String paramString): 默认从起始位置开始比较

        boolean startsWith(String paramString, int paramInt): paramInt 开始比较的位置

      

    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;
        // Note: toffset might be near -1>>>1.
        if ((toffset < 0) || (toffset > value.length - pc)) {
            return false;
        }
        while (--pc >= 0) {
            if (ta[to++] != pa[po++]) {
                return false;
            }
        }
        return true;
    }

 

    5, endsWith

       提现了方法的重用性.

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

 

     6, hashCode

      当hash为0的时候,会进行计算,

        计算方法是遍历每个字符进行 当前hash值*31+当前字符值 赋予hash 

    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];
            }
            hash = h;
        }
        return h;
    }

 

    7, indexOf   

        找出目标字符串第一次出现的位置

    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,
            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);
            }

            /* 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;
                }
            }
        }
        return -1;
    }

 

    8, substring

     截取字符串,主要是调用构造器String(val,beginIndex,subLen),

     而这个构造器用的是Arrats.copyOfRange(value, offset, offset+count);

     substring(int benginIndex,int endIndex);

     substring(int beginIndex);

    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);
    }

 

   9, concat

      拼接字符串,使用的是getChars方法

  

    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);
    }

    10, trim

      去掉首尾空格. 实现简单,不用解释

       

    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;
    }

 

   11, replace

    替换,没有使用正则表达式, replaceAll使用的是正则表达式

   

    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;
    }

 

     12, getBytes

    用的是StringCoding.encode方法,使用的是ISO-8859-1编码.

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

    static byte[] encode(char[] ca, int off, int len) {
        String csn = Charset.defaultCharset().name();
        try {
            // use charset name encode() variant which provides caching.
            return encode(csn, ca, off, len);
        } catch (UnsupportedEncodingException x) {
            warnUnsupportedCharset(csn);
        }
        try {
            return encode("ISO-8859-1", ca, off, len);
        } catch (UnsupportedEncodingException x) {
            // If this code is hit during VM initialization, MessageUtils is
            // the only way we will be able to get any kind of error message.
            MessageUtils.err("ISO-8859-1 charset not available: "
                             + x.toString());
            // If we can not find ISO-8859-1 (a required encoding) then things
            // are seriously wrong with the installation.
            System.exit(1);
            return null;