java ToString（）方法无法编译

Question

不太清楚为什么jGrasp不接受我的toString equals方法。任何帮助都会很好，陷于停顿

public class Rottweiler extends GuardDog {
    private String color = "Brown";

    public String toString() {
        String returnString = super.toString();
        returnString += String.format(Constant.FORMAT, "Color:", this.color);

        return toString;
    }
}