04_字符串连接

1.字符串连接操作符"+",不适合用在大规模字符串连接场合，由于字符串不可变，2个字符串连接在一起时，它们内容都要被拷贝。

 

测试代码：

public class _51StringConcatenation {
	private static final int NUM_ITEMS = 5000;
	private static final String LINE_FOR_ITEM = "DJLKFLDLJKFKJLDKJFKJDKFLKDLKL" +
			"LKJFLKDLKFLKJDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDLKKFDFDJFKLKLDFDFKDF";
	
	
	
	
	/**
	 * @param args
	 */
	public static void main(String[] args) {
		statement();
		statement2();
		String result = statement3();
		System.out.println(NUM_ITEMS*LINE_FOR_ITEM.length() == result.length());
	}
	
	
	private static String statement(){
		String result = "";
		long start = System.currentTimeMillis();
		
		for(int i=0;i<NUM_ITEMS;i++){
			result += LINE_FOR_ITEM;
		}
		
		System.out.println("String concatention Using Time:"+(System.currentTimeMillis() - start));
		return result;
	}
	
	private static String statement2(){
		StringBuilder result = new StringBuilder();
		long start = System.currentTimeMillis();
		
		for(int i=0;i<NUM_ITEMS;i++){
			result.append(LINE_FOR_ITEM);
		}
		
		System.out.println("String Builder Using Time:"+(System.currentTimeMillis() - start));
		return result.toString();
	}
	
	
	private static String statement3(){
		StringBuilder result = new StringBuilder(NUM_ITEMS*LINE_FOR_ITEM.length());
		long start = System.currentTimeMillis();
		
		for(int i=0;i<NUM_ITEMS;i++){
			result.append(LINE_FOR_ITEM);
		}
		
		System.out.println("String Builder When Allocat Size Using Time:"+(System.currentTimeMillis() - start));
		return result.toString();
	}


}

  打印结果:

 

String concatention Using Time:35172

String Builder Using Time:47

String Builder When Allocat Size Using Time:0

true

具有初始大小StringBuilder性能最好，其次StringBuilder，“+”号连接最差，

测试jdk1.6测试，jdk1.6在编译时，已经把"+"号连接转换成StringBuilder，不知道为何性能还是这么差