Need help understanding Booleans (setting and use)

Question

I do not understand how to set multiple boolean values in a single if statement, nor am I certain if this is possible. I attempted to write out simple test code to experiment and found that I was confusing myself more.

import java.util.Scanner;

boolean t = false;
boolean f = false;
boolean a = false;

System.out.println("Enter T or F");
Scanner scan  = new Scanner(System.in);
String t_f = scan.nextLine();

if(t_f == "t"){
     t = true;
     a =false;
}
else if(t_f == "f"){
    f = true;
    a  = true;
}

if(t = true){
    System.out.println("true");
}
else if (f = true){
    System.out.println("false");
}
else if(a = true){
    System.out.println("test");
}

I am finding that if I enter 'f', "true" is printed, which shouldn't be happening as I've set it to print false if 'f' is entered. (for example) I've attempted to manipulate the if statements varying the use of 'else if' and 'if' however I have had no success. Clearly I am missing a fundamental concept here, if anyone could help it would be greatly appreciated.

Use `str1.equals(str2)` to compare `String`s. – Christian Tapia Feb 05 '14 at 07:23 — Christian Tapia, Feb 05 '14 at 07:23

score 3 · Answer 1 · answered Feb 05 '14 at 07:23

3

You need to use == for boolean comparison(or for primitive date types). = is the assignment operator.

if(t == true){

Also, since its boolean, you don't need to compare as well.

if(t){ // if requires a boolean so just using the boolean variable would do

And use equals() method for String comparisons. == with Strings compares the object references.

if(t_f.equals("t")){

answered Feb 05 '14 at 07:23

Rahul

44,383
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-1 it's not a boolean - it's a string. – Nir Alfasi Feb 05 '14 at 07:24
@alfasin: You're looking at the wrong part of the code. There are multiple bugs. – user2357112 Feb 05 '14 at 07:24
@alfasin - Check the edited answer. `t` is boolean. And `t_f` is `String`. – Rahul Feb 05 '14 at 07:25
+1 Yeps, you are correct. – Nir Alfasi Feb 05 '14 at 07:26
After changing it to `if(t)` (as well as the rest) my program terminates without printing out anything? – Group2 Feb 05 '14 at 07:26
@user3274070: You need to fix the other bug, too. The one with the string comparisons. – user2357112 Feb 05 '14 at 07:27
@user3274070 - Make both the changes and give either "t" or "f" as the input. You'll see that it prints true or false accordingly. – Rahul Feb 05 '14 at 07:29
1

Oh whoops...silly errors in both cases. Thank you! – Group2 Feb 05 '14 at 07:29
for string comparison against constants put that constant value first to avoid nullpointers; last option then is if("t".equals(t_f)){ – hcpl Feb 05 '14 at 19:36

score 1 · Accepted Answer · edited May 23 '17 at 12:32

1

Use str1.equals(str2) to compare Strings.

if (t_f.equals("t"))

Remember that = is the assignment operator. You should use == to compare primitive types (int, boolean, float, double, ...). But since if statement expects a boolean as condition, you can simple use this:

if (t) // if (t == true)
...
if (!t) // if (t == false)

edited May 23 '17 at 12:32

Community

1
1

answered Feb 05 '14 at 07:23

Christian Tapia

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Need help understanding Booleans (setting and use)

2 Answers2