Input and Control Statements

This lesson explains how Java programs receive input and how they control the order of execution using decision statements and loops.

BufferedReader can read text from the keyboard. Since it reads input as text, numeric values must be converted before computation.

Common conversions:

When using BufferedReader, import java.io.* and handle IOException.

Scanner is commonly used for beginner Java input because it provides methods for different data types.

Values passed after the program name can be accessed through args. Since each command-line argument is a String, numeric input must be parsed.

Example run:

java CommandLineDemo Ana 19 50.5

JOptionPane from javax.swing can show simple input and message dialog boxes.

Useful methods:

Dialog message types include ERROR_MESSAGE, INFORMATION_MESSAGE, WARNING_MESSAGE, QUESTION_MESSAGE, and PLAIN_MESSAGE.

Control flow statements decide which statements run, how often they run, and when execution should stop or jump.

An if statement runs code only when a condition is true. An else block runs when the condition is false. An else if chain checks multiple conditions in order.

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

A switch statement chooses a block of code based on a single expression. It is useful when comparing one value against several exact choices.

switch (month) {
    case 1:
        System.out.println("January");
        break;
    case 2:
        System.out.println("February");
        break;
    default:
        System.out.println("Invalid month");
}

Use break to stop execution from falling into the next case.

Loops repeat code while a condition is true.

for (int i = 1; i <= 5; i++) {
    System.out.println(i);
}

int i = 1;
while (i <= 5) {
    System.out.println(i);
    i++;
}

int i = 1;
do {
    System.out.println(i);
    i++;
} while (i <= 5);

break immediately exits a loop. continue skips the remaining statements in the current iteration and proceeds to the next iteration.

Nested loops are loops placed inside other loops. They are useful for tables, patterns, and grid-like output.

Modify the grade value and observe which branch runs. Then try changing the loop limit.