If statment needing if else for fales.

Question

Just been working over a text book and came accross a problem. The question in the text book is to build a Reactor class with with two methods first been UML:-
+increaseTemperature(boolean)
+decreasTemperature(boolean)
the qustions say the increase temperature should never be allowed to rise above 10. If an attempt is made then return a value of false.
so the code i have is :-
public class Reactor {
    private int temperature;
    public final static int MAX = 10;
    public final static int MIN = -10;
    public void Reactor()
                      temperature = 0;
    public int getTemperature()
        return temperature;
    public void increaseTemperature(boolean alarm)
        if ( temperature >= MAX)
            alarm = false;
        temperature++;
        alarm = true;
    public void decreaseTemperature(boolean alarm)
        if ( temperature <= MIN)
            alarm = false;
        temperature--;
        alarm = true;
    } now for the problem I have got and don't understand why this happens. In my main method I have an if statment that works on the return of the increaseTemperature(boolean)
so if return true print reactor is fine else print reactor is critical. However this does not work but if i place if else false it does work?? see code below
public class Main {
     * @param args the command line arguments
    public static void main(String[] args) {
        char option;
        Reactor reactorSheffield = new Reactor();
boolean alarm = true;
        System.out.println("Welcome to the Reactor program for sheffield");
        System.out.println("please select for the following options ");
        do{
        System.out.println("[1] check temperature ");
        System.out.println("[2] incress the temp ");
        System.out.println("[3] decress the temp ");
        System.out.println("[4] exit the programm ");
        option = EasyScanner.nextChar();
        switch (option)
            case '1': reactorTemp(reactorSheffield);
            break;
            case '2': reactorSheffield.increaseTemperature(alarm);
            break;
        if (alarm = true)
            System.out.println("*****Reactor is fine*****"+reactorSheffield.getTemperature());
        else
            System.out.println("**********Reactor is critical******"+reactorSheffield.getTemperature());
        }while (option != '3');
    } not working
public class Main {
     * @param args the command line arguments
    public static void main(String[] args) {
        char option;
        Reactor reactorSheffield = new Reactor();
boolean alarm = true;
        System.out.println("Welcome to the Reactor program for sheffield");
        System.out.println("please select for the following options ");
        do{
        System.out.println("[1] check temperature ");
        System.out.println("[2] incress the temp ");
        System.out.println("[3] decress the temp ");
        System.out.println("[4] exit the programm ");
        option = EasyScanner.nextChar();
        switch (option)
            case '1': reactorTemp(reactorSheffield);
            break;
            case '2': reactorSheffield.increaseTemperature(alarm);
            break;
        if (alarm = true)
            System.out.println("*****Reactor is fine*****"+reactorSheffield.getTemperature());
        else if(alarm = false)
            System.out.println("**********Reactor is critical******"+reactorSheffield.getTemperature());
        }while (option != '3');
    }thsi works why?
I don't understand why the secound one works and still learning about this. Any help would be great so I understand how it all works.

Answer 1

In Java all variables are passed by value. This means that the value of the variable you are passing into the method is copied to the method. The method does not have access to the original variable reference you passed it, it will therefore not be updated.
public class PassByValue {
    public static void flipBoolean(boolean b) {
        b = !b;
    public static boolean flipBooleanWithReturn(boolean b) {
        return !b;
    public static void main(String[] args) {
        boolean testBool = true;
        System.out.printf("Original value of boolean: %s%n%n",Boolean.toString(testBool));
        flipBoolean(testBool);
        System.out.printf("Value of boolean after attempting to change variable submitted as parameter: %s%n%n",Boolean.toString(testBool));
        boolean newBool = flipBooleanWithReturn(testBool);
        System.out.printf("Value of boolean obtained through return parameter (simple and correct way to notify whether an action has been performed): %s%n%n",Boolean.toString(newBool));
}Also, you are unecessarily using an equals sign to do a logical evaluation of a boolean's value. This is causing a problem, because you are in fact using an assignment operator not a logical equals.
Look at this code for a demonstration.
public class ShowProperBooleanUsage {
    public static void main(String[] args) {
        boolean b = false;
        if(b) {
            System.out.println("This does not get executed because the value of the boolean evaluates to false");
        } else {
            System.out.println("This is the proper way to use an if statement with a boolean value");
        System.out.println();
        if(b == true) {
            System.out.println("unnecessary check for boolean equality");
        } else {
            System.out.println("There is no need to perform a logical equals against a literal boolean to evaluate the boolean value of a boolean");
            System.out.println("It is redundant just like the explanation");
            System.out.println("Saying (b == true) is exactly the same as saying b");           
            System.out.println("Saying (b == false) is exactly the same as saying !b");                       
        System.out.println();
        if(b = true) {
            System.out.println("You might think this is the same as the statement above");
            System.out.println("However, what you are really doing is setting the value of b to true and then check to see if it is true");
            System.out.println("Therefore the execution will end up in the if part of the statement instead of the else");
        } else {
            System.out.println("Doesn't get here, you have performed a logical coding error which will not be caught by the compiler");
            System.out.println("It is better practice to avoid making assignment at the same time you are performing logical evaluations to avoid these problems.");
}