Java Regex group

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class hw2 {
    
    static char[] evaluate(String input) throws IllegalArgumentException 
    {
        input = input.replaceAll("\\p{Z}", "");
        
        String result_num1 = null;
        String result_op = null;
        String result_num2 = null;
        
        Pattern pattern_num = Pattern.compile("([+-][0-9]*)([+-/*])([+-]\\d*)");
        Matcher matcher_num = pattern_num.matcher(input);
        
        while(matcher_num.find()) {
            result_num1 = matcher_num.group(1); 
            result_op = matcher_num.group(2); 
            result_num2 = matcher_num.group(3); 
        }
        
        System.out.println(result_num1);
        System.out.println(result_op);
        System.out.println(result_num2);
          
        return element_2; 
    }
  
    public static void main(String[] args) throws Exception{
        
        InputStreamReader isr = new InputStreamReader(System.in);
        BufferedReader reader = new BufferedReader(isr);
        String input = reader.readLine();
        
        char[] result = evaluate(input); 
        System.out.println(result);
        
    }  
}

Could anyone please guide on this one.

input: 300 + 25

expected output:
300
+
25
+

output:
null
null
null