[转载]Java数组扩容算法及Java对它的应用

 1 public class Arrays {
 2 /**  3  * @param original: the array to be copied  4  * @param newLength: the length of the copy to be returned  5  * @return a copy of the original array, truncated or padded with zeros  6  * to obtain the specified length  7 */  8 public static int[] copyOf(int[] original, int newLength) {  9 int[] copy = new int[newLength]; 10 System.arraycopy(original, 0, copy, 0, 11  Math.min(original.length, newLength)); 12 return copy; 13  } 14 /** 15  * @param original the array from which a range is to be copied 16  * @param from the initial index of the range to be copied, inclusive 17  * @param to the final index of the range to be copied, exclusive. 18  * (This index may lie outside the array.) 19  * @return a new array containing the specified range from the original array, 20  * truncated or padded with zeros to obtain the required length 21 */ 22 public static int[] copyOfRange(int[] original, int from, int to) { 23 int newLength = to - from; 24 if (newLength < 0) 25 throw new IllegalArgumentException(from + " > " + to); 26 int[] copy = new int[newLength]; 27 System.arraycopy(original, from, copy, 0, 28 Math.min(original.length - from, newLength)); 29 return copy; 30  } 31 }

 1 import java.util.Arrays;
 2 
 3 /** 数组变长算法!  4  * 数组对象长度不可改变  5  * 但是很多实际应用需要长度可变的数组  6  * 可以采用复制为容量更大的新数组, 替换原数组, 实现变长操作  7  * */  8 public class ArrayExpand {  9 public static void main(String[] args) { 10 //数组变长(扩容)算法! 11 int[] ary={1,2,3}; 12 ary=Arrays.copyOf(ary, ary.length+1); 13 ary[ary.length-1]=4; 14 System.out.println(Arrays.toString(ary));//[1, 2, 3, 4] 15 //字符串连接原理 16 char[] chs = { '中', '国' }; 17 chs = Arrays.copyOf(chs, chs.length + 1); 18 chs[chs.length - 1] = '北'; 19 chs = Arrays.copyOf(chs, chs.length + 1); 20 chs[chs.length - 1] = '京'; 21 //字符数组按照字符串打印 22 System.out.println(chs);//中国北京 23 //其他数组按照对象打印 24 System.out.println(ary);//[I@4f1d0d 25  } 26 }

 1 public class CountCharDemo {
 2 public static void main(String[] args) {  3 char key = '字';  4 String str = "统计一个字符在字符串中的所有位置";  5 int[] count=count(str,key);  6 System.out.println(Arrays.toString(count));//[4, 7]  7  }  8 public static int[] count(String str,char key){  9 int[] count={}; 10 for(int i=0;i<str.length();i++){ 11 char c=str.charAt(i); 12 if(c==key){ 13 //扩展数组 14 count=Arrays.copyOf(count, count.length+1); 15 //添加序号i 16 count[count.length-1]=i; 17  } 18  } 19 return count; 20  } 21 }

 1 public abstract class AbstractStringBuilder {  2 /** The value is used for character storage.*/  3 char value[];  4 /** The count is the number of characters used.*/  5 int count;  6 /** Returns the length (character count).*/  7 public int length() {  8 return count;  9  } 10 11 public AbstractStringBuilder append(String str) { 12 if (str == null) 13 str = "null"; 14 int len = str.length(); 15 if (len == 0) 16 return this; 17 int newCount = count + len; 18 if (newCount > value.length) 19  expandCapacity(newCount); 20 str.getChars(0, len, value, count); 21 count = newCount; 22 return this; 23  } 24 25 /** 26  * 自动实现Java数组扩容 27 */ 28 void expandCapacity(int minimumCapacity) { 29 int newCapacity = (value.length + 1) * 2; 30 if (newCapacity < 0) { 31 newCapacity = Integer.MAX_VALUE; 32 } else if (minimumCapacity > newCapacity) { 33 newCapacity = minimumCapacity; 34  } 35 value = Arrays.copyOf(value, newCapacity); 36  } 37 }

 1 /** 字符串数组与String类的原理 */
 2 public class CharArrayDemo {  3 public static void main(String[] args) {  4 /* Java 可以将char[]作为字符串处理 */  5 char[] ch1={'中','国','北','京'};  6 char[] ch2={'欢','迎','您'};  7 System.out.println(ch1);//中国北京  8 System.out.println(ch2);//欢迎您  9 /* char[]运算需要编程处理,如连接: */ 10 char[] ch3=Arrays.copyOf(ch1, ch1.length+ch2.length); 11 System.arraycopy(ch2, 0, ch3, ch1.length, ch2.length); 12 System.out.println(ch3);//中国北京欢迎您 13 /* String API提供了简洁的连接运算: */ 14 String str1="中国北京"; 15 String str2="欢迎您"; 16 String str3=str1.concat(str2); 17 System.out.println(str3);//中国北京欢迎您 18 /* 字符串转大写: */ 19 char[] ch4={'A','a','c','f'}; 20 char[] ch5=Arrays.copyOf(ch4, ch4.length); 21 for(int i=0;i<ch5.length;i++){ 22 char c=ch5[i]; 23 if(c>='a' && c<='z'){ 24 ch5[i]=(char)(c+('A'-'a')); 25  } 26  } 27 System.out.println(ch5);//AACF, 原数组ch4不变 28 String str4="Aacf"; 29 String str5=str4.toUpperCase();//原字符串str4保持不变 30 System.out.println(str5);//AACF 31  } 32 }