1.
package com.Test1;public class Test1 {/*** @param args*/public static void main(String[] args) {// TODO Auto-generated method stubString s1="213131";int i,num=0,j;boolean ju=true;        //判断能否输出for(i=s1.length()-1,j=1;i>=0;i--,j=j*10){num=num+(s1.charAt(i)-48)*j;        //将字符从后往前转换为整型if(s1.charAt(0)=='0'){        //判断首位是否为0System.out.println("首位为0，有误");ju=false;}    }if(ju==true)System.out.println(num);}
}2.
package com.Test2;
import java.util.Arrays;
public class Test2 {/*** @param args*/public static void main(String[] args) {// TODO Auto-generated method stubint a[]=new int[]{1,3,5,7};int b[]=new int[]{2,4,6,8};int d[]=new int[a.length+b.length];d=add(a,b);    for(int i=0;i<d.length ;i++)        //输出最终数组
        System.out.print(d[i]);}public static int[] add(int[] a,int b[]){    //静态数组不用新建对象int c[]=new int[a.length+b.length];int i,j=0;for(i=0;i<a.length;i++){        //连接两个数组c[i]=a[i];}for(;j<b.length;j++,i++){c[i]=b[j];}Arrays.sort(c);            //排序连接后的数组return c;}
}3.
package com.Test3;public class Test3 {public static void main(String[] args) {// TODO Auto-generated method stubString str="12345";if(str.isEmpty()){System.out.println("字符串为空，有误");}else{System.out.println(transform(str));}}public static String transform(String str){char ch;ch=str.charAt(0);        //剥离首字符if(1==str.length()){    //到最后一个时返回return str;}else{str=transform(str.substring(1,str.length()));    //递归，把下一级的值赋给上一级str=str+ch;        //倒转
        }return str;}
}4.
package com.emp;public class emp {public static void main(String[] args) {// TODO Auto-generated method stubint a[][]=new int[4][4];int i=0,j=0,n=1;while(i!=3||j!=3){if(i==0&&j==0){a[i][j]=n;n++;j++;}else{if(j==3){a[i][j]=n;n++;while(i!=3){i++;j--;a[i][j]=n;n++;}j++;}else if(i==3){a[i][j]=n;n++;while(j!=3){i--;j++;a[i][j]=n;n++;}i++;}else if(i==0){a[i][j]=n;n++;while(j!=0){i++;j--;a[i][j]=n;n++;}i++;} else if(j==0){a[i][j]=n;n++;while(i!=0){i--;j++;a[i][j]=n;n++;}j++;}}}a[3][3]=16;for(i=0;i<4;i++){for(j=0;j<4;j++){if(a[i][j]>9)System.out.print(a[i][j]+"  ");elseSystem.out.print(a[i][j]+"   ");}System.out.println();}}}