String with while loop not working, please help me

[Quang Tien Pham]

the code keep asking too input again, even i input a, s, c

import java.util.*;

public class test {
    public static void main(String[] args) 
    {
        String type;
        Scanner kbInput = new Scanner(System.in);
        type = kbInput.next().toLowerCase();
        do
        {
            if (type == "a" || type == "s" || type == "c" )
            {
                break;
            }
            else
            {
                System.out.println("please input again");
                type = kbInput.next().toLowerCase();
            }
        }
        while (true);  
} 
}

[Naadiyaar Mansouri]

To compare literals please use equals() method instead of ==. By using == you're actually comparing references rather than contents of variables. So modified version of your code will look like

if (type.equals("a") || type.equals("s") || type.equals("c")) {
...
}

You can find more information about the difference here:
https://www.javatpoint.com/difference-between-comparing-string-using-equals-equals-and-dot-equals-method-in-java

Edit: Welcome to the Ranch

[Campbell Ritchie]

Welcome to the Ranch

Please avoid ...while (true); that denotes an infinite loop, and, to me, an infinite loop is the epitome of a program gone wrong. If you want a do loop, write it like this:-

do
{
   // ...
} while (!input.equals("a") && !input.equals("c") &&
         !input.equals("s");

No need for break; any more. Note how you can break the line so it doesn't go over the right margin of the paper or screen.
The link you were given isn't clearly written, but it gives the correct answer, as NM told you. You can rely on == always returning false in this program. One of our people, Winston Gutkowski, wrote an FAQ about it. I like to call == the, “same object operator,” even if that isn't its official name.
If you try something cimilar in C#, you will find that == gives different results.
Don't declare the String so early; it Java® it is best to declare local variables as late as possible:-  public static void main(String[] args) {    String type;    Scanner kbInput = new Scanner(System.in);    type = kbInput.next().toLowerCase();    // ...Improved version:-public static void main(String[] args) {    Scanner kbInput = new Scanner(System.in);    String type = kbInput.next().toLowerCase();    // ...Probably better still, using var:-public static void main(String[] args) {    var kbInput = new Scanner(System.in);    var type = kbInput.next().toLowerCase();    // ...

[Carey Brown]

Campbell and I have different feelings about the use of infinite loops with break. Keyboard input methods are generally short so readability is not an issue and the loops are infinite until valid input has been entered.

// Notes
// 1) There should only ever be one Scanner created from System.in
//    so making it a constant encourages that. As a constant the
//    name KB_INPUT needs to be in all caps.
// 2) Output a prompt to let the user know it's waiting.
// 3) I find infinite loops to be acceptable in limited quantities.
//    I also prefer the while(true) to be at the top instead of a
//    'do' to let the reader know right off that it is infinite.
// 4) Get keyboard input inside the loop so you only need that
//    statement once. Use nextLine() instead of next(). When getting
//    keyboard input you are always looking for input followed by
//    pressing ENTER which is what nextLine() does.
// 5) Use equals() instead of '==' to compare Strings.
// 6) Don't need an 'else' here.
class test1
{
	public static final Scanner KB_INPUT = new Scanner( System.in ); // 1

	public static void main( String[] args )
	{
		System.out.print( "Enter text: " ); // 2
		while( true ) // 3
		{
			String type = KB_INPUT.nextLine().toLowerCase(); // 4
			if( type.equals( "a" ) || type.equals( "s" ) || type.equals( "c" ) ) // 5
			{
				break;
			}
			// 6
			System.out.println( "please input again" );
		}
	}
}

// This would be my second choice.
class test2
{
	public static final Scanner KB_INPUT = new Scanner( System.in );

	public static void main( String[] args )
	{
		boolean		finished = false;
		System.out.print( "Enter text: " );
		while( ! finished )
		{
			String type = KB_INPUT.nextLine().toLowerCase();
			if( type.equals( "a" ) || type.equals( "s" ) || type.equals( "c" ) )
			{
				finished = true;
			}
			else
			{
				System.out.println( "please input again" );
			}
		}
	}
}

// Don't like. Needs 'type' to be declared out side of the loop and
// you have two long statements comparing 'type' that have to be
// kept in sync.
class test3
{
	public static final Scanner KB_INPUT = new Scanner( System.in );

	public static void main( String[] args )
	{
		String		type;
		System.out.print( "Enter text: " );
		do
		{
			type = KB_INPUT.nextLine().toLowerCase();
			if( type.equals( "a" ) || type.equals( "s" ) || type.equals( "c" ) )
			{
				break;
			}
			else
			{
				System.out.println( "please input again" );
			}
		} while( !type.equals("a") && !type.equals("c") &&
		         !type.equals("s") );
	}
}

[Campbell Ritchie]

Carey Brown wrote:Campbell and I have different feelings about the use of infinite loops with break. . . .

We do. Be grateful for small mercies; I could have changed the loop to a plain while loop with tests for the input in the loop heading. I shall leave that delight for another occasion.

[Carey Brown]

Naadiyaar, this snippet of code you're working on is very familiar in text base user interfaces. So much so that very often a project will have a Keyboard.java file (or some other name) to collect all of those methods as static utility methods.  The methods should be written as generically as possible to allow for reuse. Taking your snippet as an example it might look like this.

public class test4
{
	public static void main( String[] args )
	{
		System.out.println(
			Keyboard.enterOneOf( "Enter text: ", "a", "s", "c" ) );
	}
}
	
// A "utility" class
class Keyboard
{
	private static final Scanner KB_INPUT = new Scanner( System.in );

	public static String enterOneOf( String prompt, String... valid )
	{
		while( true )
		{
			System.out.print( prompt );
			String input = KB_INPUT.nextLine().toLowerCase();
			for( String s : valid )
				if( s.equals( input ) )
					return input;
			System.out.println( "Invalid, please enter again" );
		}
	}
}