diff --git a/docs/JDK/String.md b/docs/JDK/String.md
index fcb5dbe..8772e1a 100644
--- a/docs/JDK/String.md
+++ b/docs/JDK/String.md
@@ -1 +1,202 @@
-努力编写中...
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+String的源码大家应该都能看懂，这里就不一一分析咯，重点讲一下equals()和hashcode()方法，然后看一下String类常用方法的实现，就当一起温习一下咯。
+```java
+public final class String
+    implements java.io.Serializable, Comparable<String>, CharSequence {
+    
+    /** 保存String的字节数组 */
+    private final char value[];
+
+    /** 缓存这个String的hash值 */
+    private int hash; // Default to 0
+
+    /** use serialVersionUID from JDK 1.0.2 for interoperability */
+    private static final long serialVersionUID = -6849794470754667710L;
+
+    /**
+     * 1、Object的 hashCode()返回该对象的内存地址编号，而equals()比较的是内存地址是否相等；
+     * 2、需要注意的是当equals()方法被重写时，hashCode()也要被重写；
+     * 3、按照一般hashCode()方法的实现来说，equals()相等的两个对象，hashcode()必须保持相等；
+     *    equals()不相等的两个对象，hashcode()未必不相等
+     * 4、一个类如果要作为 HashMap 的 key，必须重写equals()和hashCode()方法
+     */
+    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;
+    }
+    
+    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;
+    }
+
+    /**
+     * 指定下标的char
+     */
+    public char charAt(int index) {
+        if ((index < 0) || (index >= value.length)) {
+            throw new StringIndexOutOfBoundsException(index);
+        }
+        return value[index];
+    }
+
+    /**
+     * 是否以 prefix 为前缀
+     */
+    public boolean startsWith(String prefix) {
+        return startsWith(prefix, 0);
+    }
+
+    /**
+     * 是否以 suffix 为后缀
+     */
+    public boolean endsWith(String suffix) {
+        return startsWith(suffix, value.length - suffix.value.length);
+    }
+
+    /**
+     * 该String对象 是否满足 regex正则表达式
+     */
+    public boolean matches(String regex) {
+        return Pattern.matches(regex, this);
+    }
+
+    /**
+     * 字符替换
+     */
+    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;
+    }
+
+    /**
+     * 子串替换
+     */
+    public String replaceAll(String regex, String replacement) {
+        return Pattern.compile(regex).matcher(this).replaceAll(replacement);
+    }
+
+    /**
+     * 子串替换，只替换第一个
+     */
+    public String replaceFirst(String regex, String replacement) {
+        return Pattern.compile(regex).matcher(this).replaceFirst(replacement);
+    }
+
+    /**
+     * 按 regex 切割成多个子串
+     */
+    public String[] split(String regex) {
+        return split(regex, 0);
+    }
+
+    /**
+     * 剪切指定范围的字符串
+     */
+    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);
+    }
+
+    public String substring(int beginIndex, int endIndex) {
+        if (beginIndex < 0) {
+            throw new StringIndexOutOfBoundsException(beginIndex);
+        }
+        if (endIndex > value.length) {
+            throw new StringIndexOutOfBoundsException(endIndex);
+        }
+        int subLen = endIndex - beginIndex;
+        if (subLen < 0) {
+            throw new StringIndexOutOfBoundsException(subLen);
+        }
+        return ((beginIndex == 0) && (endIndex == value.length)) ? this
+                : new String(value, beginIndex, subLen);
+    }
+
+    /**
+     * 获取该String 对应的 char[]
+     */
+    public char[] toCharArray() {
+        // Cannot use Arrays.copyOf because of class initialization order issues
+        char result[] = new char[value.length];
+        System.arraycopy(value, 0, result, 0, value.length);
+        return result;
+    }
+
+    /**
+     * 大小写转换
+     */
+    public String toLowerCase() {
+        return toLowerCase(Locale.getDefault());
+    }
+    public String toUpperCase() {
+        return toUpperCase(Locale.getDefault());
+    }
+
+    /**
+     * str在本String对象中第一次出现的下标
+     */
+    public int indexOf(String str) {
+        return indexOf(str, 0);
+    }
+
+    /**
+     * str在本String对象中最后一次出现的下标
+     */
+    public int lastIndexOf(String str) {
+        return lastIndexOf(str, value.length);
+    }
+}
+```
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