如果使用Java语句，则无法获得其他语句

Question

Ok I am trying to make this simple thing but it won't work. I am a beginner in Java and would like some help. Every time I run the code below I get the output That is not a valid option . What am I doing wrong?

 package test;

 import java.util.Scanner;

 public class options {
     public void options() {
         Scanner scnr = new Scanner(System.in);
         String slctn;

         System.out.println("What would you like to do?");
         System.out.println("a) Travel the expedition");
         System.out.println("b) Learn more about the expedition");

         slctn = scnr.nextLine();
         if (slctn == "a"){
             travel exeTravel = new travel();
             exeTravel.travel();
         }else if (slctn=="b"){
             learn exeLearn = new learn();
             exeLearn.learn();
         }else{
             System.out.println("That is not a valid option");
         }
     } 
 }

Answer 1

Well, first off, == is a fundamental operator in the language. The result type of the expression is a boolean. For comparing boolean types, it compares the operands for the same truth value. For comparing reference types, it compares the operands for the same reference value (ie, refer to the same object or are both null). For numeric types, it compares the operands for the same integer value or equivalent floating point values. See the Java Language Specification .

In contrast, equals() is an instance method which is fundamentally defined by the java.lang.Object class. This method, by convention, indicates whether the receiver object is "equal to" the passed in object. The base implementation of this method in the Object class checks for reference equality. Other classes, including those you write, may override this method to perform more specialized equivalence testing. See the Java Language Specification .

The typical "gotcha" for most people is in using == to compare two strings when they really should be using the String class's equals() method. From above, you know that the operator will only return "true" when both of the references refer to the same actual object. But, with strings, most uses want to know whether or not the value of the two strings are the same -- since two different String objects may both have the same (or different) values.

     slctn = scnr.nextLine();
     if (slctn.equals("a")){
         travel exeTravel = new travel();
         exeTravel.travel();
     }else if (slctn.equals("b")){
         learn exeLearn = new learn();
         exeLearn.learn();
     }else{
         System.out.println("That is not a valid option");
     }

Answer 2

slctn.equals("a") will work.

Read this to understand why: What is difference between == and equals() in java?

Answer 3

In Java, when you need to compare two objects for equality (that is, to determine if they have the same value) you must use equals() . The == operator is used for testing if two objects are identical , that is: if they're exactly the same object in memory. In your code, replace this:

slctn == "a"
slctn == "b"

With this:

"a".equals(slctn)
"b".equals(slctn)

Also notice that it's a good idea to invert the order of the comparison ( "a" before slctn ), just in case slctn is null.

Answer 4

In java when matching any object the == operator will only match the reference of those two objects.

If we take your example slctn == "a". Say slctn has its reference value at abc123, your other sting "a" will have a different reference value as it is not the same object.

The method .equals checks what the letters in the string object are and matches the value of the letters in the two strings. Therefore if your object slctn contains "a", it will match with the string "a"

Answer 5

In java == operator compare reference of two objects, for sample :

String s_1 = new String("Sample");
String s_2 = new String("Sample");

System.out.println(s_1 == s_2);

result will is :

false

this happen because s_1 is a reference at memory and s_2 is difference refernce at memroy also. For solve this issue , you have to compare tow objects by equals method. for sample

String s_1 = new String("Sample");
String s_2 = new String("Sample");

System.out.println(s_1.equals(s_2));

result will is :

true