为什么我的Java switch语句不起作用？

Question

I am trying to do the following for a Java switch method with a series of JUnit Asserts but am stuck on using "less than" and "greater than" for two cases (see string/int error below), and am not sure how to use the ">" and "<" in my cases.

Here is the exercise followed by my code, followed by the error.

/*  
Create a method which uses a switch statement to return a String representing the int passed in as a parameter to the method:
            • given 1 return "One"
            • given 2 return "Two"
            • given 3 return "Three"
            • given 4 return "Four"
            • given an integer > 4, return "Too big"
            • given an integer < 1, return "Too small"
*/
@Test
public void switchIntExample() {
    assertEquals("One", stringRepInt(1));
    assertEquals("Two", stringRepInt(2));
    assertEquals("Three", stringRepInt(3));
    assertEquals("Four", stringRepInt(4));
    assertEquals("Too big.", stringRepInt(>4));      
//        assertEquals("Too small.", stringRepInt(<4));
}

//Switch statement for above:
public String stringRepInt(int numberSize) {

    String numVar = null;

    switch (numberSize) {
        case 1:
            numVar = "One";
            break;
        case 2:
            numVar = "Two";
            break;
        case 3:
            numVar = "Three";
            break;
        case 4:
            numVar = "Four";
            break;           
     //TODO:  question on how to do LESS THAN and GREATER THAN:
     // error line: 
     case  (numVar > 4):
            numVar = "Too big.";
            break;

        default:
            break;
    }
    System.out.println(numVar);
    return numVar;
}

ERROR: Error:(293, 27) java: bad operand types for binary operator '>' first type: java.lang.String second type: int

Answer 1

case statements only support constant expressions (you cannot do less than, or greater than, in a case and you can't test numVar - the String - with less than). You can use an if and something like

public String stringRepInt(int numberSize) {
    String numVar = null;
    if (numberSize > 4) {
         numVar = "Too big.";
    } else {
        switch (numberSize) {
        case 1:
            numVar = "One";
            break;
        case 2:
            numVar = "Two";
            break;
        case 3:
            numVar = "Three";
            break;
        case 4:
            numVar = "Four";
            break;       
        default:
            break;
    }
    System.out.println(numVar);
    return numVar;
}

Answer 2

You should add it to the default section, so like this:

default:
    numVar = "Too Big";
    break;

Answer 3

The purpose of the default section is to deal with all cases not dealt with by the switch cases. You should be taking advantage of that (as @GreenMatt and @FredK mentioned in comments) by putting the 'if checks' in the default section, as follows:

public String stringRepInt(int numberSize) {

    String numVar = null;

    switch (numberSize) {
        case 1:
            numVar = "One";
            break;
        case 2:
            numVar = "Two";
            break;
        case 3:
            numVar = "Three";
            break;
        case 4:
            numVar = "Four";
            break;
        default:
            if(numberSize > 4)
                numVar = "Too big";
            break;
    }
    System.out.println(numVar);
    return numVar;
}

Further, you could add else if (numberSize < 1) numVar = "Too small"; under the if statement if you want to check for number's smallest than one. This is also important because it prevents your method from returning null . (Which currently happens if the user enters a value less than 1)

The resulting code is as follows:

public String stringRepInt(int numberSize) {

    String numVar = null;

    switch (numberSize) {
        case 1:
            numVar = "One";
            break;
        case 2:
            numVar = "Two";
            break;
        case 3:
            numVar = "Three";
            break;
        case 4:
            numVar = "Four";
            break;
        default:
            if(numberSize > 4)
                numVar = "Too big";
            else
                numVar = "Too small";
            break;
    }
    System.out.println(numVar);
    return numVar;
}

Answer 4

In the error line you are comparing String and int.

error line: case (numVar > 4): numVar = "Too big."; break;

you have declared numVar as String and comparing that with int value 4. Please correct that or I think you are trying to compare numberSize with value 4. Convert that String to int and compare it.