Best approach to converting Boolean object to string in java

If you are sure that your value is not null you can use third option which is

String str3 = b.toString();

and its code looks like

public String toString() {
    return value ? "true" : "false";
}

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If you want to be null-safe use String.valueOf(b) which code looks like

public static String valueOf(Object obj) {
    return (obj == null) ? "null" : obj.toString();
}

so as you see it will first test for null and later invoke toString() method on your object.

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Calling Boolean.toString(b) will invoke

public static String toString(boolean b) {
    return b ? "true" : "false";
}

which is little slower than b.toString() since JVM needs to first unbox Boolean to boolean which will be passed as argument to Boolean.toString(...), while b.toString() reuses private boolean value field in Boolean object which holds its state.

public class Sandbox {

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        Boolean b = true;
        boolean z = false;
        echo (b);
        echo (z);
        echo ("Value of b= " + b +"\nValue of z= " + z);
    }

    public static void echo(Object obj){
        System.out.println(obj);
    } 

}

Result
--------------
true
false
Value of b= true
Value of z= false
--------------

I don't think there would be any significant performance difference between them, but I would prefer the 1st way.

If you have a Boolean reference, Boolean.toString(boolean) will throw NullPointerException if your reference is null. As the reference is unboxed to boolean before being passed to the method.

While, String.valueOf() method as the source code shows, does the explicit null check:

public static String valueOf(Object obj) {
    return (obj == null) ? "null" : obj.toString();
}

Just test this code:

Boolean b = null;

System.out.println(String.valueOf(b));    // Prints null
System.out.println(Boolean.toString(b));  // Throws NPE

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For primitive boolean, there is no difference.

If this is for the purpose of getting a constant "true" value, rather than "True" or "TRUE", you can use this:

Boolean.TRUE.toString();
Boolean.FALSE.toString();