jdk类库源码分析-String类

骑猪看日落 2022-07-15 11:19 282阅读 0赞

从今天开始研究jdk基础类库的源码，从中学习优秀的设计思想，提高自己的能力。至于怎么查看源码，这里不多说，网上很多教程，直接上干货儿，欢迎大家提出宝贵意见，共同学习，一起进步。（这里我研究的是jdk1.7）  
**String源码分析**  
**一 、String类**  
在java.lang包中，此类被final修饰，表示String的对象是不可变量。String类实现了Serizlizable，Comparable, CharSequence接口。  
Serizlizable接口没有任何方法和域，仅用于标识序列化的语意，实现此接口的类是可序列化的，是java提供的通用数据保存和读取的接口。  
Comparable接口只有一个compareTo(T o)方法。  
![这里写图片描述][20161115230708250]  
CharSequence接口有length(),charAt(int index),subSequence(int start,int end)，toString()方法。  
**二、String属性**  
String类中包含一个不可变的char数组用来存放字符串，一个int型用来存放计算后的哈希值。  
![这里写图片描述][20161115231150018]  
**三、String类常用方法**

package jdk;
    
    import java.io.UnsupportedEncodingException;
    import java.nio.charset.Charset;
    import java.util.Arrays;
    
    public class Stringstr {
        //使用字符数组存放字符串
        private final char value[];
        //存放哈希值
        private int hash; 
    
        private static final long serialVersionUID = -6849794470754667710L;
        //不含参数的构造函数
        public String() {
            this.value = new char[0];
        }
        //String类型参数
        public String(String original) {
                this.value = original.value;
                this.hash = original.hash;
        }
        //参数类型是char类型数组，使用的是Arrays类复制
        public String(char value[]) {
                this.value = Arrays.copyOf(value, value.length);
        }
        //从位置offset开始复制count个字符
        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);
            }
        //构建一个子字符集
         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;
    
                // Pass 1: Compute precise size of char[]
                int n = count;
                for (int i = offset; i < end; i++) {
                    int c = codePoints[i];
                    if (Character.isBmpCodePoint(c))
                        continue;
                    else if (Character.isValidCodePoint(c))
                        n++;
                    else throw new IllegalArgumentException(Integer.toString(c));
                }
    
                // Pass 2: Allocate and fill in char[]
                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;
            }
         //检查字节数组是否越界
         private static void checkBounds(byte[] bytes, int offset, int length) {
                if (length < 0)
                    throw new StringIndexOutOfBoundsException(length);
                if (offset < 0)
                    throw new StringIndexOutOfBoundsException(offset);
                if (offset > bytes.length - length)
                    throw new StringIndexOutOfBoundsException(offset + length);
            }
    
         //StringBuffer类型转为String使用同步关键字，线程安全
         public String(StringBuffer buffer) {
                synchronized(buffer) {
                    this.value = Arrays.copyOf(buffer.getValue(), buffer.length());
                }
          }
         //StringBuilder类型转为String
          public String(StringBuilder builder) {
                this.value = Arrays.copyOf(builder.getValue(), builder.length());
            }
    
          //返回字符串长度
          public int length() {
                return value.length;
            }
          //判断是否为空
          public boolean isEmpty() {
                return value.length == 0;
            }
          //字符串存储使用字符数组，使用索引返回char值
          public char charAt(int index) {
                if ((index < 0) || (index >= value.length)) {
                    throw new StringIndexOutOfBoundsException(index);
                }
                return value[index];
            }
          //返回指定索引处的字符,unicode字符
          public int codePointAt(int index) {
                if ((index < 0) || (index >= value.length)) {
                    throw new StringIndexOutOfBoundsException(index);
                }
                return Character.codePointAtImpl(value, index, value.length);
            }
          //返回指定索引之前的字符
          public int codePointBefore(int index) {
                int i = index - 1;
                if ((i < 0) || (i >= value.length)) {
                    throw new StringIndexOutOfBoundsException(index);
                }
                return Character.codePointBeforeImpl(value, index, 0);
            }
          //返回指定处的unicode代码数
          public int codePointCount(int beginIndex, int endIndex) {
                if (beginIndex < 0 || endIndex > value.length || beginIndex > endIndex) {
                    throw new IndexOutOfBoundsException();
                }
                return Character.codePointCountImpl(value, beginIndex, endIndex - beginIndex);
            }
          //和一个对象比较
          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) {
                        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;
            }
          //两个字符串相互比较
          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) {
                        return c1 - c2;
                    }
                    k++;
                }
                return len1 - len2;
            }
          //哈希码计算
          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;
            }
          //从指定索引开始是否是指定字符串
          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;
            }
          //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);
                return new String(buf, true);
            }
         //替换字符
          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;
            }
          //去掉空格
          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;
            }
    
    }

总结：在这里我主要是拿出了常用的几个方法，实际上String源码中有更多的方法，有兴趣的朋友可以自己看看。以前自己使用String调用这些方法的时候并没有考虑过怎么个实现，现在看完这些之后，发现底层包含了很多的算法和数据结构在其中，收获很大。

[20161115230708250]: /images/20220715/3246e0f593af4af38da5c66871ace6ed.png
[20161115231150018]: /images/20220715/5106ff4689d644229a1d12c1168990aa.png