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Условия и цикли в Java

Intro

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Control flow blocks are basically blocks of code that control the way data flows, or else which statements are executed when. This can be useful if you only want certain areas of your code to be executed under a given condition.

There are two major types of decision structures: conditionals and loops. Code contained in a conditional block may or may not be executed; the decision is made at runtime based on a given condition. Code contained in a loop is repeated; how many times is also decided at runtime.

The boolean Primitive

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Java has a specific kind of variable for determining whether something is true or false, called a boolean. A boolean type can be defined with a boolean literal (true or false), or by evaluating an expression with a boolean operator.

Boolean Operators

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These can be used to check if something is true or false. Most take numeric variables (byte, short, int, long, float, or double) and evaluate to a boolean. Note that booleans are variables. Here is a list of all boolean operators:

Symbol Desc. Syntax Example
< Less than [variable] < [variable] if(i1 < i2)
> Greater than [variable] > [variable] if(i1 > i2)
!= Not equal [v] != [v] if(i1 != i2)
== Equal [v] == [v] if(i1 == i2)
<= and >= Less than or equal/Greater than or equal [v] >= or <= [v] if(i1 >= i2) (or other way)
|| Or boolean || boolean if(i1 == 5 || i1 > 8)
&& And b && b while(i1<0 && i2>5)
! Not !Boolean if(!b1)


As you can see above, and, or, and not are the only operators that work on booleans. These, essentially, create new booleans. For example, if you said: 

boolean b1 = false;
boolean b2 = true;
boolean b3 = b1 || b2;
boolean b4 = b1 && b2;

In this example, b3 would be true (because one of b1 and b2 are true) and b4 would be false (because both b1 and b2 are not true).

Finally, you can do: 

int i1=5;
int i2=5;
int i3=7;
int i4=3;
boolean b1 = i1 < i2;                         // i1 is not less than i2 (both are 5), so b1 is false
boolean b2 = i1 == i2 && i3 > i2;             // b2 is true because i1 is equal to i2, and i3 is greater than i2
boolean b3 = i4 < i3 && i1 > i3;              // b3 is false - i4 is less than i3, but i1 is NOT greater than i3
boolean b4 = i4 < i3 || i1 > i3;              // b4 is true because i4 < i3. Only one is needed because it is an ||
boolean b5 = (i4 < i3 && i3 < i2) || i3 > i1; // See below for info on parentheses
boolean b6 = !true;                           // This is false because !true is false, and !false is true.

Expressions within parenthesis are always evaluated first, from inside to outside. Ample use of parentheses can greatly clarify your intent. For example, consider the following code: 

boolean a = true;
boolean b = true;
boolean c = false;

boolean result = a || b && c;

What value is stored in result? If the or is evaluated first, then the result is false, because the code would evalute to 

a || b && c
true || true && false
true && false
false

If the and is evaluated first, though, the result is true: 

a || b && c
true || true && false
true || false
true

There are rules governing which operators are evaluated first. You could remember that logical and has a higher precedence than logical or (that is, and is evaluated first), you could look it up, or you could just use parentheses! Then you don't have to bother memorizing operator precedence, or waste time checking a table.

Besides, what if you wanted the or evaluated first? You're sunk... or you could add parentheses to ensure that the or is first: 

boolean result = (a || b) && c;

This will force the or to evaluate first, and be sure that anyone reading your code will know what you mean.

You will learn below how these boolean operators can be used inside control flow blocks.


If/Else

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You have seen this statement used in the previous section: 

if (ourShip.speed<10) ourShip.mainThrustPulse(2);

"ourShip.speed<10" is a boolean. If it is true, then it will do what comes after. Note that ourShip.speed and "10" are not booleans. The < makes a boolean if the speed is less than 10. If you did the following: 

if (true) ourShip.mainThrustPulse(2)

Guess what - it would automatically go to the preceding statement. Note that this can only be used for one statement. If you want to execute multiple statement for a given condition you can do: 

if(CONDITION)
{//THEN...
   DO SOMETHING;
   DO SOMETHING2;
}

Everything within { and } is executed if CONDITION is true. Now, suppose you want multiple if statements. You could just put 2 in a row, but if the first one makes the second impossbile, then you can do: 

if(CONDITION)
{
   DO SOMETHING;
}
else if(ANOTHER CONDITION)
{
   DO SOMETHING ELSE;
}

Now, if you want to say "If the first statement is false, then do..." You could do: 

else if(!CONDITION)
{
   DO SOMETHING;
}

But a better way is: 

else
{
   DO SOMETHING;
}

Back to our ship. Lets make an if/else block that does: "If the ship's speed is less than 10, fire 2 pulses. If it is less than 15, fire one. Otherwise, lower the speed by firing backwards (-1)" 

if (ourShip.speed < 10)
{
   ourShip.mainThrustPulse(2);
}
else if(ourShip.speed < 15)
{
   ourShip.mainThrustPulse(1);
}
else
{
   ourShip.mainThrustPulse(-1);//Slow down
}

public class Multiplying { 

 public static void main(String args[])
 {
   int a = 6;
   int b = a*a*a*a;
   System.out.println(b + "");
 }

}



Answer: 

if(i1 == i2)
{
   System.out.println("Equal");
}

else
{
   System.out.println("Not equal");
}

Some examples of the usage of else/if: 

public class Examples {
        public static void main (String[] args) {
	
		int i1 = 7;
	
	//1. Check if an integer is equal to 1
      
      if(i1 == 1){
      	System.out.println("Step 1 - Equal");
      	}
      else System.out.println("Step 1 - Not Equal");
      	
      //2. Check if an integer is equal to 6 then 7
      
      if(i1==6){
      	System.out.println("Step 2 - Equal #1");
      		}
      else if(i1==7){
      	
      	System.out.println("Step 2 - Equal #2");
      	}
      else System.out.println("Step 2 - Not Equal");
}
}

If you compile this, the result should be:

Step 1 - Not Equal

Step 2 - Equal #2


Note that any code for the "Ship" cannot be executed because that class does not exist and neither does the value "speed". You can make it a class, though, when you learn about Classes and Objects.

Switch

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Switch and case are the integer equivalent of if/else statements. They are simple to understand. As usual, the syntax: 

switch(SOME_INT)
{
   case X:
      doSomething;
      break;

   case Y: 
      doSomething2;
      break;

   default:
      doSomething3;
      break;
}

X and Y are numbers. doSomething 1, 2, and 3 are all statements. When SOME_INT is equal to X or Y, doSomething or doSomething2 is executed, respectively. Default means that if SOME_INT is not equal to either X or Y, do doSomething3. Note that you can have more or less cases.

Here is the equivalent in if/else statements 

if(SOME_INT == X)
{doSomething;}
else if(SOME_INT == Y)
{doSomething2;}
else
{doSomething3}

Read "break" for more info on the break statements. If you did not have it in this program, after it executed doSomething, it would also execute doSomething2 and doSomething3 till it runs into a break statement. Usually, you do not want this to happen. However, sometimes you might.

While Loops

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Loops will execute a block of code while a given condition is true. Basically, that block of code is executed until the condition is false. The condition is checked after the block of code is executed each time, and also before it is executed the first time. The basic syntax: 

while(CONDITION)
{
   DO SOMETHING;
}

So, you can guess that: 


while(true)
{
   DO SOMETHING;
}

will execute DO SOMETHING forever. The statment within the ( and ) is like that of an if/else statement.

Exercise 1: Design a while block that executes the if/else if/else code for the ship in the If/Else Statements section forever.

Exercise 2: Change the above so that it stops after 10 times. You will need to know the < operator: using the if statment, "if(SOMETHING < SOMETHING2) {...}" is valid. Basically, if s is less than s2 the next part will execute. You should use int variables for this exercise.



Answer to exercise 2: 

int times=0;
while(times < 10)
{
   //Put if/else code here...
   times++;//Equivalent of times = times + 1;
}

For Loop

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A for loop is a neat package for executing some statements a set number of times over a certain variable. The syntax is fairly simple. First, there is the initialization, next is the loop end test, and finally is the increment step. This is written as follows: 

for (int i = 0; i < someNum; i++)
{
     // do something
}

In this example, we declare an integer (int) named i and initialize it to 0 in the initialization step. In the loop end test, we check to see that the value of i is strictly less then whatever the value of someNum is. Finally, the increment step says i++, which is a shorthand for i=i+1, or add 1 to i.

There is another for loop more recently introduced to Java made for iteration (moving one by one through a list). It uses any type of list structure as its base. An example is this: 

int[] integerArray = {2,3,4,5}; // an integer array is a basic list of integers
LinkedList<Integer> listOfNumbers = new LinkedList<Integer>(); //Don't worry about this syntax now

for (int i : integerArray){
     i = i+1;
     listOfNumbers.add(i);
}
//integerArray now equals [2,3,4,5]
//listOfNumbers now equals [3,4,5,6]

In this example we are iterating an array named integerArray. The for loop will run 4 times, each time with "i" equaling a different value in the array. When the items in the list structure (like the example above) are primitive values (byte, int, long, float, double, boolean, char...) the value of "i" will just be a copy. This is why the values inside of integerArray are not changed even though "i" is changed inside the loop.

If, however, the values in the list are references, they will be directly under the influence of any changes occurring inside the loop as in this example: 

//Note, this code will not compile with a working Ship class
LinkedList<Ship> listOfShips = new LinkedList<Ship>(); //Don't worry about this syntax now
listOfShips.add(new Ship(Color.RED));
listOfShips.add(new Ship(Color.BLUE));
listOfShips.add(new Ship(Color.GREEN));

LinkedList<Ship> newListOfShips = new LinkedList<Ship>(); //Don't worry about this syntax now

for (Ship shipToChange : listOfShips){
     shipToChange.setColor(Color.BLACK);
     newListOfShips.add(shipToChange);
}
//all ships in listOfShips now have color black
//all ships in newListOfShips now have color black

In this example we start out with a list named listOfShips holding three "ships" all of different colors. The for loop iterated through the list and changed each ships's color to black, then it added the ship to another list called newListOfShips. Since shipToChange is a reference to a variable instead of a variable itself, changes to it affect the items in listOfShips.

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