数字与字符串系列教材 （十）- 自己开发一个Java StringBuffer

StringBuffer第二道练习题答案

步骤1:IStringBuffer接口
步骤2:value和capacity
步骤3:带参构造方法
步骤4:反转 reverse
步骤5:插入insert 和 append
步骤6:删除 delete
步骤7:练习-性能比较
步骤8:答案-性能比较

步骤 1 : IStringBuffer接口

package character;    public interface IStringBuffer {     public void append(String str); //追加字符串      public void append(char c);  //追加字符     public void insert(int pos,char b); //指定位置插入字符     public void insert(int pos,String b); //指定位置插入字符串     public void delete(int start); //从开始位置删除剩下的     public void delete(int start,int end); //从开始位置删除结束位置-1     public void reverse(); //反转     public int length(); //返回长度 }

步骤 2 : value和capacity

value：用于存放字符数组
capacity： 容量
无参构造方法： 根据容量初始化value

public MyStringBuffer(){     value = new char[capacity]; }

package character; public class MyStringBuffer implements IStringBuffer{     int capacity = 16;     int length = 0;     char[] value;     public MyStringBuffer(){         value = new char[capacity];     }           @Override     public void append(String str) {         // TODO Auto-generated method stub               }     @Override     public void append(char c) {         // TODO Auto-generated method stub               }     @Override     public void insert(int pos, char b) {         // TODO Auto-generated method stub               }     @Override     public void delete(int start) {         // TODO Auto-generated method stub               }     @Override     public void delete(int start, int end) {         // TODO Auto-generated method stub               }     @Override     public void reverse() {         // TODO Auto-generated method stub               }     @Override     public int length() {         // TODO Auto-generated method stub         return 0;     } }

步骤 3 : 带参构造方法

package character; public class MyStringBuffer implements IStringBuffer{     int capacity = 16;     int length = 0;     char[] value;     public MyStringBuffer(){         value = new char[capacity];     }           //有参构造方法     public MyStringBuffer(String str){         if(null!=str)             value =str.toCharArray();                   length = value.length;                   if(capacity<value.length)             capacity  = value.length*2;     }           @Override     public void append(String str) {         // TODO Auto-generated method stub               }     @Override     public void append(char c) {         // TODO Auto-generated method stub               }     @Override     public void insert(int pos, char b) {     }     @Override     public void delete(int start) {         // TODO Auto-generated method stub               }     @Override     public void delete(int start, int end) {         // TODO Auto-generated method stub               }     @Override     public void reverse() {         // TODO Auto-generated method stub               }     @Override     public int length() {         // TODO Auto-generated method stub         return length;     }     @Override     public void insert(int pos, String b) {     } }

步骤 4 : 反转 reverse

package character; public class MyStringBuffer implements IStringBuffer {     int capacity = 16;     int length = 0;     char[] value;     public MyStringBuffer() {         value = new char[capacity];     }     // 有参构造方法     public MyStringBuffer(String str) {         this();         if (null == str)             return;         if (capacity < str.length()) {             capacity = value.length * 2;             value = new char[capacity];         }         if (capacity >= str.length())             System.arraycopy(str.toCharArray(), 0, value, 0, str.length());         length = str.length();     }     @Override     public void reverse() {         for (int i = 0; i < length / 2; i++) {             char temp = value[i];             value[i] = value[length - i - 1];             value[length - i - 1] = temp;         }     }     @Override     public void append(String str) {         // TODO Auto-generated method stub     }     @Override     public void append(char c) {         // TODO Auto-generated method stub     }     @Override     public void insert(int pos, char b) {         // TODO Auto-generated method stub     }     @Override     public void insert(int pos, String b) {         // TODO Auto-generated method stub     }     @Override     public void delete(int start) {         // TODO Auto-generated method stub     }     @Override     public void delete(int start, int end) {         // TODO Auto-generated method stub     }     @Override     public int length() {         // TODO Auto-generated method stub         return length;     }     public String toString() {         char[] realValue = new char[length];         System.arraycopy(value, 0, realValue, 0, length);         return new String(realValue);     }     public static void main(String[] args) {         MyStringBuffer sb = new MyStringBuffer("there light");         sb.reverse();         System.out.println(sb);     } }

步骤 5 : 插入insert 和 append

边界条件判断
插入之前，首先要判断的是一些边界条件。 比如插入位置是否合法，插入的字符串是否为空

扩容
1. 要判断是否需要扩容。 如果插入的字符串加上已经存在的内容的总长度超过了容量，那么就需要扩容。
2. 数组的长度是固定的，不能改变的，数组本身不支持扩容。 我们使用变通的方式来解决这个问题。
3. 根据需要插入的字符串的长度和已经存在的内容的长度，计算出一个新的容量。 然后根据这个容量，创建一个新的数组，接着把原来的数组的内容，复制到这个新的数组中来。并且让value这个引用，指向新的数组，从而达到扩容的效果。

插入字符串
1. 找到要插入字符串的位置，从这个位置开始，把原数据看成两段，把后半段向后挪动一个距离，这个距离刚好是插入字符串的长度
2. 然后把要插入的数据，插入这个挪出来的，刚刚好的位置里。

修改length的值
最后修改length的值，是原来的值加上插入字符串的长度

insert(int, char)
参数是字符的insert方法，通过调用insert(int, String) 也就实现了。

append
追加，就是在最后位置插入。 所以不需要单独开发方法，直接调用insert方法，就能达到最后位置插入的效果

package character;    public class MyStringBuffer implements IStringBuffer{        int capacity = 16;     int length = 0;     char[] value;     public MyStringBuffer(){         value = new char[capacity];     }            //有参构造方法     public MyStringBuffer(String str){         this();         if(null==str)             return;                    if(capacity<str.length()){             capacity  = value.length*2;             value=new char[capacity];         }                    if(capacity>=str.length())             System.arraycopy(str.toCharArray(), 0, value, 0, str.length());                    length = str.length();                }            @Override     public void append(String str) {         insert(length,str);     }        @Override     public void append(char c) {         append(String.valueOf(c));                }        @Override     public void insert(int pos, char b) {         insert(pos,String.valueOf(b));     }        @Override     public void delete(int start) {         // TODO Auto-generated method stub                }        @Override     public void delete(int start, int end) {         // TODO Auto-generated method stub                }        @Override     public void reverse() {         for (int i = 0; i < length/2; i++) {             char temp = value[i];             value[i] = value[length-i-1];             value[length-i-1] = temp;         }     }        @Override     public int length() {         // TODO Auto-generated method stub         return length;     }        @Override     public void insert(int pos, String b) {            //边界条件判断         if(pos<0)             return;                    if(pos>length)             return;                    if(null==b)             return;                    //扩容         while(length+b.length()>capacity){             capacity = (int) ((length+b.length())*1.5f);             char[] newValue = new char[capacity];             System.arraycopy(value, 0, newValue, 0, length);             value = newValue;         }                    char[] cs = b.toCharArray();                    //先把已经存在的数据往后移                    System.arraycopy(value, pos, value,pos+ cs.length, length-pos);         //把要插入的数据插入到指定位置         System.arraycopy(cs, 0, value, pos, cs.length);                    length = length+cs.length;                }            public String toString(){                    char[] realValue = new char[length];            System.arraycopy(value, 0, realValue, 0, length);                    return new String(realValue);                }            public static void main(String[] args) {         MyStringBuffer sb = new MyStringBuffer("there light");         System.out.println(sb);         sb.insert(0, "let ");         System.out.println(sb);            sb.insert(10, "be ");         System.out.println(sb);         sb.insert(0, "God Say:");         System.out.println(sb);         sb.append("!");         System.out.println(sb);         sb.append('?');         System.out.println(sb);         sb.reverse();         System.out.println(sb);                }    }

步骤 6 : 删除 delete

package character; public class MyStringBuffer implements IStringBuffer{     int capacity = 16;     int length = 0;     char[] value;     public MyStringBuffer(){         value = new char[capacity];     }           //有参构造方法     public MyStringBuffer(String str){         this();         if(null==str)             return;                   if(capacity<str.length()){             capacity  = value.length*2;             value=new char[capacity];         }                   if(capacity>=str.length())             System.arraycopy(str.toCharArray(), 0, value, 0, str.length());                   length = str.length();               }           @Override     public void append(String str) {         insert(length,str);     }     @Override     public void append(char c) {         append(String.valueOf(c));               }     @Override     public void insert(int pos, char b) {         insert(pos,String.valueOf(b));     }     @Override     public void delete(int start) {                   delete(start,length);     }     @Override     public void delete(int start, int end) {         //边界条件判断         if(start<0)             return;                   if(start>length)             return;                   if(end<0)             return;                   if(end>length)             return;                   if(start>=end)             return;                   System.arraycopy(value, end, value, start, length- end);         length-=end-start;               }     @Override     public void reverse() {         for (int i = 0; i < length/2; i++) {                           char temp = value[i];             value[i] = value[length-i-1];             value[length-i-1] = temp;         }               }     @Override     public int length() {         // TODO Auto-generated method stub         return length;     }     @Override     public void insert(int pos, String b) {         //边界条件判断         if(pos<0)             return;                   if(pos>length)             return;                   if(null==b)             return;                   //扩容         while(length+b.length()>capacity){             capacity = (int) ((length+b.length())*1.5f);             char[] newValue = new char[capacity];             System.arraycopy(value, 0, newValue, 0, length);             value = newValue;         }                   char[] cs = b.toCharArray();                   //先把已经存在的数据往后移                   System.arraycopy(value, pos, value,pos+ cs.length, length-pos);         //把要插入的数据插入到指定位置         System.arraycopy(cs, 0, value, pos, cs.length);                   length = length+cs.length;               }           public String toString(){                   char[] realValue = new char[length];         System.arraycopy(value, 0, realValue, 0, length);                   return new String(realValue);               }           public static void main(String[] args) {         MyStringBuffer sb = new MyStringBuffer("there light");         System.out.println(sb);         sb.insert(0, "let ");         System.out.println(sb);         sb.insert(10, "be ");         System.out.println(sb);         sb.insert(0, "God Say:");         System.out.println(sb);         sb.append("!");         System.out.println(sb);         sb.append('?');         System.out.println(sb);         sb.reverse();         System.out.println(sb);                   sb.reverse();         System.out.println(sb);                   sb.delete(0,4);         System.out.println(sb);         sb.delete(4);         System.out.println(sb);     } }

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