public class Search{
public static void main(String [] args) {
int a[] = {1, 3, 4, 7, 9, 20, 58, 79 };
int i = -1;
System.out.println(twofen(a,i));
}  

public static int twofen(int []a,int num) {
if(a.length == 0)return -1;
int firstpos = 0;
int lastpos = a.length -1;
while(firstpos <=lastpos){
if(num == a[(firstpos + lastpos) / 2])return (firstpos + lastpos) / 2;
if(num < a[(firstpos + lastpos) / 2]){lastpos = ((firstpos + lastpos) / 2)-1;}
if(num > a[(firstpos + lastpos) / 2]){firstpos = ((firstpos + lastpos) / 2)+1;}

}
return -1;
}
}

------解决方案--------------------
将if(num  < a[m]){ 
    lastpos = a[m]-1; 
 } 
 if(num >  a[m]){ 
    firstpos = a[m]+1; 
 }两名改为： 
 if(num  < a[m]){ 
    lastpos = m-1; 
 } 
 if(num >  a[m]){ 
    firstpos = m+1; 
 } 
  


------解决方案--------------------
还有你好像多了一个大括号

------解决方案--------------------
1，“int twofen(int []a,int num)”应将此函数应放在class Search里，而不是外面
2，“if(num < a[m]){lastpos = a[m]-1;}”须将 a[m]-1改为m-1
   及“if(num > a[m]){firstpos = a[m]+1;}“里的a[m]+1改为m+1即可

------解决方案--------------------
Java code
public class Search{
    public static void main(String [] args) {
        int a[] = {1, 3, 4, 7, 9, 20, 58, 79 };
        int i = 9;
        System.out.println(twofen(a,i));
    }  
    public static int twofen(int []a,int num) {
        if(a.length == 0) return -1;
        int firstpos = 0;
        int lastpos = a.length -1;
        int m = (firstpos + lastpos) / 2;
        while(firstpos <= lastpos){
            if(num == a[m]) return m;
            if(num < a[m]){    lastpos = m-1;}
            if(num > a[m]){    firstpos = m+1;}
            m = (firstpos + lastpos) / 2;
        }
        return -1;
    }
}