如何重写compareTo方法

Question

here is my compareTo method, but im still getting "missing return statement" warning. can anyone tell me what is wrong with my code?

    public int compareTo(Flows other) {
    if(this.srcAddr.equals(other.srcAddr)){
        if(this.dstAddr.equals(other.dstAddr)){
                 if(this.srcPort.equals(other.srcPort)){
                     if(this.dstPort.equals(other.dstPort)){
                         if(this.protocol.equals(other.protocol)){
                             return 0;
                         }
                     }
                 }
         }
 }
}

Answer 1

Two things:

• You get the "missing return statement" because there are paths of execution where no value is returned. For example, when the first if statement computes to false.

• You are breaking the compareTo() contract. For the following call: a.compareTo(b), the result should be: 0 if a equals b, <0 if a is minor than b, and >0 if a is greater than b. It seems you're using the compareTo() to check for equality, in that case the correct approach is overriding the equals() method.

Answer 2

This looks like an equals method. If the intention simply is to compare if the two are the same, I would do something like

return srcAddr.equals(other.srcAddr) &&
       dstAddr.equals(other.dstAddr) &&
       srcPort.equals(other.srcPort) &&
       dstPort.equals(other.dstPort) &&
       protocol.equals(other.protocol);

If it's not the intention , you're probably breaking the contract of compareTo since your method doesn't seem to adhere to the transitivity requirement . From the docs of Comparable :

The implementor must also ensure that the relation is transitive

Answer 3

It's because there's a possibility in your code for the compareTo to return nothing! Think about if all of those if statements fail, then it will hit the end of the method and not have returned anything. You need a return further down:

public int compareTo(Flows other) {
  if(this.srcAddr.equals(other.srcAddr)){
      if(this.dstAddr.equals(other.dstAddr)){
          if(this.srcPort.equals(other.srcPort)){
              if(this.dstPort.equals(other.dstPort)){
                  if(this.protocol.equals(other.protocol)){
                      return 0;
                  }
              }
          }
      }
  }
  return 1;

}

Also you are not doing a complete compare. You need to return 0 if they are equal, less than 0 if the difference is less than and greater than 0 if it's greater. It seesm you'd be better off with overriding equals!

Maybe something like:

public boolean equals(Flows other) {
    return (this.srcAddr.equals(other.srcAddr) && this.dstAddr.equals(other.dstAddr) && this.srcPort.equals(other.srcPort) && this.dstPort.equals(other.dstPort) && this.protocol.equals(other.protocol));

Answer 4

只需在函数末尾添加“ return 1”（或其他任何东西），它就可以解决问题。

Answer 5

This will compile and run, but what about the rest of the contract? Where's less than and greater than?

public int compareTo(Flows other) {

    int value = 0;

    if(this.srcAddr.equals(other.srcAddr)){
        if(this.dstAddr.equals(other.dstAddr)){
                 if(this.srcPort.equals(other.srcPort)){
                     if(this.dstPort.equals(other.dstPort)){
                         if(this.protocol.equals(other.protocol)){
                             value = 0;
                         }
                     }
                 }
         }

    return value;
 }