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Conditionals and Control Flow

else Statement

The else statement executes a block of code when the condition inside the if statement is false. The else statement is always the last condition.

boolean condition1 = false;


if (condition1){
    System.out.println("condition1 is true");
}
else{
    System.out.println("condition1 is not true");
}
// Prints: condition1 is not true

else if Statements

else-if statements can be chained together to check multiple conditions. Once a condition is true, a code block will be executed and the conditional statement will be exited.

There can be multiple else-if statements in a single conditional statement.

int testScore = 76;
char grade;


if (testScore >= 90) {
  grade = 'A';
} else if (testScore >= 80) {
  grade = 'B';
} else if (testScore >= 70) {
  grade = 'C';
} else if (testScore >= 60) {
  grade = 'D';
} else {
  grade = 'F';
}


System.out.println("Grade: " + grade); // Prints: C

if Statement

An if statement executes a block of code when a specified boolean expression is evaluated as true.

if (true) {
	System.out.println("This code executes");
}
// Prints: This code executes


if (false) {
	System.out.println("This code does not execute");
}
// There is no output for the above statement

Nested Conditional Statements

A nested conditional statement is a conditional statement nested inside of another conditional statement. The outer conditional statement is evaluated first; if the condition is true, then the nested conditional statement will be evaluated.

boolean studied = true;
boolean wellRested = true;


if (wellRested) {
  System.out.println("Best of luck today!");  
  if (studied) {
    System.out.println("You are prepared for your exam!");
  } else {
    System.out.println("Study before your exam!");
  }
}


// Prints: Best of luck today!
// Prints: You are prepared for your exam!

NOT Operator

The NOT logical operator is represented by !. This operator negates the value of a boolean expression.

boolean a = true;
System.out.println(!a); // Prints: false


System.out.println(!false) // Prints: true

AND Operator

The AND logical operator is represented by &&. This operator returns true if the boolean expressions on both sides of the operator are true; otherwise, it returns false.

System.out.println(true && true); // Prints: true
System.out.println(true && false); // Prints: false
System.out.println(false && true); // Prints: false
System.out.println(false && false); // Prints: false

The OR Operator

The logical OR operator is represented by ||. This operator will return true if at least one of the boolean expressions being compared has a true value; otherwise, it will return false.

System.out.println(true || true); // Prints: true
System.out.println(true || false); // Prints: true
System.out.println(false || true); // Prints: true
System.out.println(false || false); // Prints: false

Conditional Operators - Order of Evaluation

If an expression contains multiple conditional operators, the order of evaluation is as follows: Expressions in parentheses -> NOT -> AND -> OR.

boolean foo = true && (!false || true); // true
/* 
(!false || true) is evaluated first because it is contained within parentheses. 


Then !false is evaluated as true because it uses the NOT operator. 


Next, (true || true) is evaluation as true. 


Finally, true && true is evaluated as true meaning foo is true. */

Compare Object References

Boolean expressions allow us to compare object references. A Boolean expression is a Java expression that, when evaluated, returns a Boolean value: true or false.

a.equals(b)
a.equals(b) && b.equals(c)

Comparing Primitive Values

We can use relational operators, such as == and !=, to compare primitive and reference values. 

class ComparingPrimitives {
  public static void main(String[] args) {
    System.out.println("Comparing ints:");
    System.out.println(4 == 5); // print false
    System.out.println(4 != 5); // print true
    System.out.println(4 == 4); // print true


    System.out.println("Comparing chars:");
    System.out.println('a' == 'b'); // print false
    System.out.println('a' != 'b'); // print true
    System.out.println('a' == 'a'); // print true
  }
}

Object Reference Aliases

An alias means that more than one reference is tied to the same object.

class ComparingAliases {
 public static void main(String[] args) {
   String farmAnimal1 = new String("cat");
   String farmAnimal2 = new String("cow");
   // farmAnimal3 references the same object as farmAnimal2
   String farmAnimal3 = farmAnimal2;
 
   // comparing different objects
   System.out.println(farmAnimal1 == farmAnimal2); // print false
   // comparing object aliases
   System.out.println(farmAnimal2 == farmAnimal3); // print true
 }
}

Comparing Object Reference Aliases

We can compare object reference values can be compared, using == and !=, to identify aliases.

class ComparingAliases {
 public static void main(String[] args) {
   String farmAnimal1 = new String("cat");
   String farmAnimal2 = new String("cow");
   String farmAnimal3 = farmAnimal2;
 
   // comparing different objects
   System.out.println(farmAnimal1 == farmAnimal2); // print false
   // comparing object aliases
   System.out.println(farmAnimal2 == farmAnimal3); // print true
 }
}

Comparing Reference Values with Null

We can compare a reference value with null, using == or !=, to determine if the reference actually references an object.

class Main
{
   public static void main(String[] args)
   {
     String word = null;
     // checking that `word != null` avoids NullPointerException error
     if (word != null && word.indexOf("a") >= 0)
     {
         System.out.println(word + " contains an a.");
     }
   }
}

Custom Class Equals Method

Classes often have their own equals method, which can be used to determine whether two objects of the class are equivalent.

class Pet {
  public String name;
  public String breed;
  public Pet (String name, String breed) {
    this.name = name;
    this.breed = breed;
  }
  // custom `equals()` method
  public boolean equals(Pet p) {
    return (p.name == name && p.breed == breed);
  }
  public static void main(String[] args) {
    Pet pet1 = new Pet("Air Bud", "Golden Retriever");
    Pet pet2 = new Pet("Air Bud", "Golden Retriever");
    // compare with `==`
    System.out.println(pet1 == pet2); 
    // compare with `.equals()`
    System.out.println(pet1.equals(pet2)); 
  }
}

DeMorgan’s Laws

DeMorgan’s Laws can be used to rewrite expressions complex boolean expressions.

The first law states that two expressions that are negated together and compared using && is equivalent to two separately negated expressions compared with ||.

The second law states that two expressions that are compared with || and are negated together are equivalent to two separately negated expressions compared with &&. 

int a = 2;
int b = 3;


boolean exp1 = !(a > b && a == b);
// rewrite using first law
exp1 = !(a > b) || !(a == b);


boolean exp2 = !(a < b || a != b);
// rewrite using second law
exp2 = !(a < b) && !(a != b);

Equivalent Boolean Expressions

Equivalent boolean expressions are separate boolean expressions that always result in the same value.

If we were to replace a boolean expression in a program with an equivalent boolean expression, there would be no impact on the output of the program.

int a = 1;
int b = 2;
// the followning expressions are equivalent
boolean exp1 = !(a == b && b >= a);
boolean exp2 = !(a == b) || !(b >= a);
boolean exp3 = a != b || a < b;


System.out.println(exp1); // Prints: true
System.out.println(exp2); // Prints: true
System.out.println(exp3); // Prints: true

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