Looping Control Statements

• Looping control statements are also called iteration statements.
• These statements are used to execute a block of statements repeatedly.
• An expression is required for looping control statements which returns boolean value.
• Block notation is mandatory when looping block has more than one statement.
• Block notation is optional when looping block has only one statement.
• When you declare any variable within loop block, that variable can be accessed only within that block. It cannot be accessed outside that block.

2.9.2.1 for Statement

Syntax

for (<initialization>; <condition>; <updation>) {
    // Statements
}

Processing Flow of for Statement

Key Points

• Initialization statement will be executed only once (at the beginning).
• Condition and updation statements can be executed multiple times.

Lab186.java

class Lab186 {

  public static void main(String[] args) {
    System.out.println(1);
    System.out.println(2);
    System.out.println(3);
    System.out.println(4);
    System.out.println(5);
    System.out.println(6);
    System.out.println(7);
    System.out.println(8);
    System.out.println(9);
    System.out.println(10);
  }
}

Lab187.java

class Lab187 {

  public static void main(String[] args) {
    for (int i = 1; i <= 10; i++) {
      System.out.println(i);
    }
  }
}

Lab188.java

class Lab188 {

  public static void main(String[] args) {
    for (int i = 1; i <= 100; i++) {
      if (i % 2 == 0) System.out.println(i);
    }
  }
}

Lab189.java

class Lab189 {

  public static void main(String[] args) {
    int n = 5;

    for (int i = 1; i <= 10; i++) System.out.println(
      n + " * " + i + " = " + (n * i)
    );
  }
}

Lab190.java

class Lab190 {

  public static void main(String[] args) {
    for (int i = 1; i <= 10; i++) {
      int a = 99;
      a++;
      System.out.println(i + "\t" + a);
    }
  }
}

Lab191.java

class Lab191 {

  public static void main(String[] args) {
    for (int i = 1; i <= 10; i++) System.out.println(i);

    System.out.println(i); // Compilation Error
  }
}

Lab192.java

class Lab192 {

  public static void main(String[] args) {
    int i = 1;

    for (; i <= 10; i++) System.out.println(i);

    System.out.println("After: " + i);
  }
}

Lab193.java

class Lab193 {

  public static void main(String[] args) {
    for (;;) {
      System.out.println("JavaWorld");
    }
  }
}

👉 This is an infinite loop

LabLab194.java

class Lab194 {
    public static void main(String[] args) {
        for (int i = 1,char ch = 'A';i<=10;i++) {
            System.out.println(i);
        }
    }
}

Compilation Error

(Invalid variable declaration inside for-loop initialization)

Key Concepts

Difference between normal print & loop

• Lab186 → manual printing
• Lab187 → same using loop (efficient)

Loop Use Cases

• Print numbers → 1 to 10
• Print even numbers → i % 2 == 0
• Multiplication table → n * i

Variable Scope in Loop

• Variable declared inside loop → not accessible outside → (Lab191 error)

Infinite Loop

for(;;)

• No condition → always true → infinite execution

Flexible for-loop syntax

for (;condition;update)

• Initialization can be outside (Lab192)

Important Rule

• In for(initialization) → variables must be of same type
• Cannot mix:

int i=1,char ch='A'//  invalid

Lab195.java

class Lab195 {

  public static void main(String[] args) {
    int a;
    char ch;
    float f;
    String str;

    for (
      a = 5, ch = 'A', f = 123456.78F, str = "";
      a >= 0;
      a--, ch++, f /= 10, str = str + ch
    ) {
      System.out.println(a + "\t" + ch + "\t" + f + "\t" + str);
    }
  }
}

Lab196.java

class Lab196 {

  public static void main(String[] args) {
    int i = 1;

    for (
      System.out.println("Start"), System.out.println("Begin");
      i <= 5;
      i++, System.out.println("Updating"), System.out.println("Updating")
    ) {
      System.out.println(i);
    }
  }
}

Lab197.java

class Lab197 {
    public static void main(String[] args) {

        for (int i = 0,System.out.println("Begin");
        i<5;
        i++,System.out.println("Updating")) {

            System.out.println(i);
        }
    }
}

Lab198.java

class Lab198 {

  public static void main(String[] args) {
    boolean b1 = true;

    for (; b1; ) {
      System.out.println("JavaWorld");
    }

    System.out.println("AFTER");
  }
}

Lab199.java

class Lab199 {

  public static void main(String[] args) {
    final boolean b1 = true;

    for (; b1; ) {
      System.out.println("JavaWorld");
    }

    System.out.println("AFTER");
  }
}

Key Concepts

Multiple Variables in for-loop

for(a=5,ch='A',f=...,str="")

• You can declare/update multiple variables
• All updates run in each iteration

Multiple Statements in Initialization & Update

for(System.out.println("Start"),System.out.println("Begin"); ...)

• Allowed using comma (,)
• Execution order is left → right

Important Restriction (Lab197 )

int i=0,System.out.println("Begin")

• Invalid
• Initialization must contain only variable declarations or expressions, not mixed improperly

Infinite Loop using boolean

boolean b1=true;
for(;b1; )

• Runs forever because condition is always true

Compile-Time Infinite Loop (Important)

final boolean b1=true;
for(;b1; )

• Compiler knows condition is always true
• So next line:

System.out.println("AFTER");

Unreachable statement → Compilation Error

Difference Summary

Case	Behavior
boolean b1 = true	Runtime infinite loop
final boolean b1 = true	Compile-time error (unreachable code)

Execution Flow Example (Lab196)

1. Initialization → prints:
   • Start
   • Begin
2. Loop starts
3. Each iteration:
   • Print i
   • Print "Updating" twice
   • Increment i

Lab200.java

class Lab200 {

  public static void main(String[] args) {
    byte b = 127;
    b++; // overflow
    System.out.println(b);

    for (byte by = 10; by > 0; by++) {
      System.out.println(by);
    }
  }
}

Lab201.java

class Lab201 {

  public static void main(String[] args) {
    int val = 2147483640;

    for (; val > 200; val++) {
      System.out.println(val);
    }

    System.out.println("After Loop :" + val);
  }
}

Lab202.java

Output Pattern

A
A A
A A A
A A A A
A A A A A

Code

class Lab202 {

  public static void main(String[] args) {
    char ch = 'A';

    for (int rows = 0; rows < 5; rows++) {
      for (int cols = 0; cols <= rows; cols++) {
        System.out.print(ch + " ");
      }

      System.out.println();
    }
  }
}

2.9.2.2 Enhanced For Statement

• It is a new feature added in Java 5
• It is also called for-each loop
• Mainly used to access elements from array/collection sequentially

Syntax

for (<dataType>varName :array|collection) {
// Statements (BLOCK)
}

Key Concepts

✔ Byte Overflow (Lab200)

• byte range → 128 to 127

byte b=127;
b++;// becomes -128

✔ Loop with byte (Important ⚠️)

for(byte by=10;by>0;by++)

• Infinite loop risk due to overflow behavior

✔ Integer Overflow (Lab201)

• int max → 2147483647
• After exceeding → wraps to negative value

✔ Nested Loop Pattern (Lab202)

• Outer loop → controls rows
• Inner loop → controls columns
• Used for pattern printing

✔ Enhanced for-loop (for-each)

Example:

int arr[]= {1,2,3,4};

for (int x :arr) {
    System.out.println(x);
}

✔ When to Use for-each

• When you only need to read values
• Not suitable when:
  • You need index
  • You want to modify elements

2.9.2.3 while Statement

Syntax

<initialization>

while (<condition>) {
// Statements (LOOP BODY)
<updation>
}

✔ Important Rule

• Condition in while statement is mandatory
• It must be of boolean type

Processing Flow of while Statement

Lab203.java

class Lab203 {

  public static void main(String[] args) {
    int i = 1;

    while (i <= 10) {
      System.out.println(i);
      i++;
    }
  }
}

Lab204.java

class Lab204 {

  public static void main(String[] args) {
    int i = 0;

    while (i++ < 10) {
      System.out.println(i);
    }
  }
}

Key Concepts

✔ Structure Difference from for-loop

for-loop	while-loop
All in one line	Separated
for(init; cond; update)	init + while(cond) + update

✔ Execution Flow

1. Initialization → runs once
2. Condition → checked
3. If true → loop body executes
4. Update happens
5. Repeat until condition becomes false

✔ Important Behavior (Lab204)

while(i++<10)

• Uses post-increment
• First compares, then increments

Output:

1
2
3
...
10

✔ Key Difference Example

Expression	Meaning
i++ < 10	compare then increment
++i < 10	increment then compare

Lab205.java

class Lab205 {
    public static void main(String[] args) {
        while() {
            System.out.println("JavaWorld");
        }
    }
}

Result:

• Compile-time error
• Condition is mandatory → cannot leave while() empty

Lab206.java

class Lab206 {

  public static void main(String[] args) {
    while (true) {
      System.out.println("JavaWorld");
    }
  }
}

✔ Output:

JavaWorld
JavaWorld
JavaWorld
...

• Infinite loop

Lab207.java

class Lab207 {

  public static void main(String[] args) {
    while (true) {
      System.out.println("JavaWorld");
    }
    System.out.println("AFTER");
  }
}

Result:

• Compile-time error
• AFTER is unreachable statement

Lab208.java

class Lab208 {

  public static void main(String[] args) {
    boolean b1 = true;
    while (b1) {
      System.out.println("JavaWorld");
    }
    System.out.println("AFTER");
  }
}

✔ Output:

JavaWorld
JavaWorld
JavaWorld
...

• Infinite loop (since b1 is always true)
• AFTER never executes

Lab209.java

class Lab209 {

  public static void main(String[] args) {
    int n = 5,
      i = 0;

    while (i++ < 10) {
      System.out.println(n * i);
    }
  }
}

✔ Output:

5
10
15
20
25
30
35
40
45
50

Lab210.java

class Lab210 {

  public static void main(String[] args) {
    int n = 5,
      i = 0;

    while (i++ < 10) System.out.println(n * i);
  }
}

✔ Output:

5
10
15
20
25
30
35
40
45
50

• {} optional for single statement

Lab211.java

class Lab211 {

  public static void main(String[] args) {
    char ch = 'A';
    int rows = 0;

    while (rows < 5) {
      int cols = 0;

      while (cols <= rows) {
        System.out.print(ch + " ");
        cols++;
      }

      System.out.println();
      rows++;
    }
  }
}

✔ Output:

A
A A
A A A
A A A A
A A A A A

🔑 Key Takeaways

✔ While Loop Rules

• Condition must be boolean
• Empty condition → compile error
• while(true) → infinite loop

✔ Infinite Loop + Code After

while(true) { }
System.out.println("Hello");

➡ Unreachable statement error

✔ Post Increment in Condition

while(i++<10)

• First compare → then increment
• Very common interview trap ⚠️

✔ Nested while loop

• Used for patterns (like Lab211)
• Outer loop → rows
• Inner loop → columns

2.9.2.4 do-while Statement

✔ When you are using for statement or while statement, the condition is verified before executing the block.

✔ So in the case of for and while, if the condition is false the first time, the block will not be executed.

✔ for and while statements are also called Entry Controlled Loop.

✔ If you want to execute the block at least once, then use do-while statement.

---

Using do-while loop

✔ In do-while, first the block of statements will be executed, and then the condition is verified.

✔ do-while statement is also called an Exit Controlled Loop.

---

Processing Flow of do-while statement

Flow Explanation:

1. Initialization
2. Execute Loop Body
3. Perform Update
4. Check Condition
   • If true → repeat loop
   • If false → STOP

---

Equivalent Structure

do {
    // Statements (Loop Body)
}while (condition);

---

Key Difference (Important for Interviews)

Loop Type	Condition Check	Executes at least once?
while	Before loop	Not guaranteed
for	Before loop	Not guaranteed
do-while	After loop	Always once

---

Diagram (do-while)

Lab212.java

class Lab212 {

  public static void main(String[] args) {
    int a = 0;

    do {
      System.out.println("Value of a: " + a);
      a--;
    } while (a > 20);
  }
}

✔ Output:

Value of a: 0

• Loop executes once (do-while guarantee), then condition becomes false

---

Lab213.java

class Lab213 {

  public static void main(String[] args) {
    int i = 1;

    do {
      System.out.println(i);
      i++;
    } while (i <= 10);
  }
}

✔ Output:

1
2
3
4
5
6
7
8
9
10

---

Lab214.java

class Lab214 {

  public static void main(String[] args) {
    int i = 0;

    do {
      System.out.println(i++);
    } while (true);
  }
}

✔ Output:

0
1
2
3
...

• Infinite loop

---

Lab215.java

class Lab215 {

  public static void main(String[] args) {
    int i = 0;

    do {
      System.out.println(i++);
    } while (true);

    System.out.println("AFTER");
  }
}

Result:

• Compile-time error
• "AFTER" is unreachable statement

---

Lab216.java

class Lab216 {

  public static void main(String[] args) {
    char ch = 'A';
    int rows = 0;

    do {
      int cols = 0;

      do {
        System.out.print(ch + " ");
        cols++;
      } while (cols <= rows);

      System.out.println();
      rows++;
    } while (rows < 5);
  }
}

✔ Output:

A
A A
A A A
A A A A
A A A A A

---

Key Takeaways

✔ do-while Behavior

• Executes at least once (even if condition is false)
• Condition is checked after execution

---

✔ Infinite Loop Case

do {
    // code
}while(true);

➡ Runs forever unless break is used

---

✔ Unreachable Code

do {
}while(true);

System.out.println("Hello");

➡ Compile-time error

---

✔ Nested do-while

• Useful for pattern printing
• Outer loop → rows
• Inner loop → columns