System.out.println("a等于b：" + (a==b));中为什么要有+

public class HelloWorld{
    public static void main(String[] args) {
  int a=16;
  double b=9.5;
  String str1="hello";
  String str2="imooc";
  System.out.println("a等于b：" + (a==b));
  System.out.println("a大于b：" + (a>=b));
  System.out.println("a小于等于b：" + (a<=b));
  System.out.println("str1等于str2：" + (str1   ==str2));
 }
}