java将去除多余只留一个空格

package test;

import java.text.DecimalFormat;

public class Test {

/**

* @param args

*/

public static void main(String[] args) {

String aa = "a b c D";

String bb = aa.substring(0,1);

String cc = bb;

for(int i = 1;i< aa.length();i++){

if(bb.equals(aa.substring(i,i+1))){

if(!bb.equals(" "))

cc += aa.substring(i,i+1);

}

else{

cc += aa.substring(i,i+1);

}

bb = aa.substring(i,i+1);

}

System.out.println(cc.trim());

}

}