谁帮我改一下，我想能做如“-5+7”的运算，然后还要能循环运算的，出来结果后能重新提示用户输入算式的：
package phrase1;
import java.io.*;
public class Stringcalcul {
public static void main(String args[])throws IOException{
String teststr;
BufferedReader buf;
String A="";
String B="";
float numA,numB,result;
System.out.print("please input:");
buf=new BufferedReader(new InputStreamReader(System.in));

teststr=buf.readLine();
char[] testchar=teststr.toCharArray();
//char[] oper={'+','-','*','/'};
for(int i=0;i<testchar.length;i++){
switch(testchar[i]){
case '+':
for(int j=0;j<i;j++){
Character a=new Character(testchar[j]);
A+=a.toString();
}
for(int j=i+1;j<testchar.length;j++){
Character b=new Character(testchar[j]);
B+=b.toString();
}
numA=Float.parseFloat(A);
numB=Float.parseFloat(B);
result=numA+numB;
System.out.println(teststr+"="+result);
break;
case '-':
for(int j=0;j<i;j++){
Character a=new Character(testchar[j]);
A+=a.toString();
}
for(int j=i+1;j<testchar.length;j++){
Character b=new Character(testchar[j]);
B+=b.toString();
}
numA=Float.parseFloat(A);
numB=Float.parseFloat(B);
result=numA-numB;
System.out.println(teststr+"="+result);
break;
case '*':
for(int j=0;j<i;j++){
Character a=new Character(testchar[j]);
A+=a.toString();
}
for(int j=i+1;j<testchar.length;j++){
Character b=new Character(testchar[j]);
B+=b.toString();
}
numA=Float.parseFloat(A);
numB=Float.parseFloat(B);
result=numA*numB;
System.out.println(teststr+"="+result);
break;
case '/':
for(int j=0;j<i;j++){
Character a=new Character(testchar[j]);
A+=a.toString();
}
for(int j=i+1;j<testchar.length;j++){
Character b=new Character(testchar[j]);
B+=b.toString();
}
numA=Float.parseFloat(A);
numB=Float.parseFloat(B);
if(numB==0){
System.out.println("the second num can't be 0");
}else{
result=numA/numB;
System.out.println(teststr+"="+result);
}
break;
}
}
}
}

我觉得应该先判断一下第一个操作数的正负号情况，然后就好判断操作符和第二个操作数的情况了。

而且每一个case都重复做了求操作数和进行运算的事情，这些都可以放在后面做的，你的switch只是用来求操作符的，建议换个思路，这样的思路如果遇到操作数有负数的情况不好办

package src;

import java.io.*;
public class Stringcalcul {
public static void main(String args[])throws IOException{
String teststr;
BufferedReader buf;
String A="";
String B="";
String keep="";
boolean keepOn=true;
float numA,numB,result;
while(keepOn){
System.out.print("please input:");
buf=new BufferedReader(new InputStreamReader(System.in));

teststr=buf.readLine();
char[] testchar=teststr.toCharArray();
//char[] oper={'+','-','*','/'};
for(int i=0;i<testchar.length;i++){
switch(testchar[i]){
case '+':
for(int j=0;j<i;j++){
Character a=new Character(testchar[j]);
A+=a.toString();
}
for(int j=i+1;j<testchar.length;j++){
Character b=new Character(testchar[j]);
B+=b.toString();
}
numA=Float.parseFloat(A);
numB=Float.parseFloat(B);
result=numA+numB;
System.out.println(teststr+"="+result);
break;
case '-':
for(int j=0;j<i;j++){
Character a=new Character(testchar[j]);
A+=a.toString();
}
for(int j=i+1;j<testchar.length;j++){
Character b=new Character(testchar[j]);
B+=b.toString();
}
numA=Float.parseFloat(A);
numB=Float.parseFloat(B);
result=numA-numB;
System.out.println(teststr+"="+result);
break;
case '*':
for(int j=0;j<i;j++){
Character a=new Character(testchar[j]);
A+=a.toString();
}
for(int j=i+1;j<testchar.length;j++){
Character b=new Character(testchar[j]);
B+=b.toString();
}
numA=Float.parseFloat(A);
numB=Float.parseFloat(B);
result=numA*numB;
System.out.println(teststr+"="+result);
break;
case '/':
for(int j=0;j<i;j++){
Character a=new Character(testchar[j]);
A+=a.toString();
}
for(int j=i+1;j<testchar.length;j++){
Character b=new Character(testchar[j]);
B+=b.toString();
}
numA=Float.parseFloat(A);
numB=Float.parseFloat(B);
if(numB==0){
System.out.println("the second num can't be 0");
}else{
result=numA/numB;
System.out.println(teststr+"="+result);
}
break;
}
}
System.out.println("Continue?(y/n)");
keep=buf.readLine();
if(keep.equals("n")) keepOn=false;
}
}
}
红色字体部分是让该程序能够循环运行的，每执行好一次运算，会提示用户是否继续，输入y，继续，输入n，结束程序。