JavaCodeOptimization 01

来源：互联网发布：java构造器是什么编辑：程序博客网时间：2024/06/06 05:04

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package java_code_optimization;import java.util.ArrayList;public class JavaCodeOptimization{public static void main(String[] args){//----------------------------------------------------------------------------------------------// demo01   字符串final类所有方法final？java编译器，提高性能。//----------------------------------------------------------------------------------------------String str = "hello world";Integer x = 1;System.out.println(x);//----------------------------------------------------------------------------------------------// demo02 字符串//----------------------------------------------------------------------------------------------//public final class StringBuilder extends AbstractStringBuilderStringBuilder sb = new StringBuilder();sb.append(str);sb.append(", java code optimization.");System.out.println(sb.toString());//----------------------------------------------------------------------------------------------// demo03//----------------------------------------------------------------------------------------------String[] arrStr = new String[10];arrStr[0] = "0";arrStr[1] = "1";arrStr[2] = "2";arrStr[3] = "3";arrStr[4] = "4";arrStr[5] = "5";int len = arrStr.length;for (int i = 0; i < len; i++){System.out.println(arrStr[i]);}//----------------------------------------------------------------------------------------------// demo04//----------------------------------------------------------------------------------------------ArrayList<String> al = new ArrayList<String>();al.add("ddd");len = al.size();for (int i = 0; i < len; i++){System.out.println(al.get(i));}//----------------------------------------------------------------------------------------------// demo05//----------------------------------------------------------------------------------------------//05 把原始数组src从下标srcPos开始的长度为len的元素 拷贝  目标数组dest的起始位置destPos//public static void arraycopy(Object src, int srcPos, Object dest, int destPos, int length)//public static void arraycopy(Object src, int srcPos, int length, Object dest, int destPos)int[] arrInt = {77, 88, 99, 1, 2, 3, 1000};System.arraycopy(arrInt, 0, arrInt, 4, 2);//{77, 88, 99, 1, 77, 88, 1000};  2,3 被77,88替换//越界会怎么样，不知道System.arraycopy(arrInt, 0, arrInt, 4, 6);//{77, 88, 99, 1, 77, 88, 1000};  2,3 被77,88替换////Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException//at java.lang.System.arraycopy(Native Method)//at java_code_optimization.JavaCodeOptimization.main(JavaCodeOptimization.java:46)}}

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