Java Basic Programming Guide

Java Basic Programming Guide | Generated by AI

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Here’s a focused guide for Part II: Basic Programming of 08553 Introduction to Programming—Java, tailored for the National Higher Education Self-study Examination (自考). This section trains you to write small, correct Java programs and understand how Java handles data and expressions.

1. Reading Input from the Console

1.1 Using `Scanner`

Java reads input with the Scanner class in java.util.

import java.util.Scanner;

public class InputExample {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.print("Enter your name: ");
        String name = sc.nextLine();
        System.out.print("Enter your age: ");
        int age = sc.nextInt();
        System.out.println("Hello " + name + ", you are " + age + " years old.");
        sc.close();
    }
}

Key points:

Always import java.util.Scanner;
Use nextInt(), nextDouble(), nextLine() based on data type.
Close the Scanner to free resources.
Be careful: nextLine() reads the rest of a line, so mixing it with nextInt() may cause skipped input.

2. Identifiers, Variables, Expressions, Assignments, and Constants

2.1 Identifiers

Names you give to variables, methods, or classes. Rules:

Must start with a letter, _, or $.
Cannot start with a number.
Case-sensitive (score ≠ Score).
Cannot be a keyword (int, class, if, etc.).

Examples: studentName, _total, $price

2.2 Variables

A variable holds data of a certain type. Declaration examples:

int age = 20;
double price = 12.5;
char grade = 'A';
boolean passed = true;

2.3 Assignment Statements

Assign a value using = (right → left):

x = 5;
y = x + 2;

2.4 Constants

Declared with final, cannot change later:

final double PI = 3.14159;

Use uppercase names for constants.

3. Numeric Data Types and Operations

3.1 Common Numeric Types

byte (8-bit integer)
short (16-bit)
int (32-bit, most common)
long (64-bit)
float (32-bit decimal)
double (64-bit decimal, more precise)

Example:

int count = 5;
double price = 19.99;

3.2 Basic Arithmetic Operators

+, -, *, /, %

Examples:

int a = 10, b = 3;
System.out.println(a / b);  // 3 (integer division)
System.out.println(a % b);  // 1
System.out.println((double)a / b); // 3.3333 (type conversion)

4. Type Conversion (Casting)

4.1 Automatic Conversion (Widening)

Small → large type automatically: int → long → float → double

Example:

int i = 10;
double d = i;  // automatic conversion

4.2 Manual Conversion (Casting)

Explicitly convert larger → smaller type:

double d = 9.7;
int i = (int) d; // i = 9 (fraction lost)

Be careful with precision loss.

5. Expression Evaluation and Operator Precedence

5.1 Order of Operations

Java follows a defined order:

Parentheses ( )
Unary +, -, ++, --
Multiplication, division, modulus * / %
Addition and subtraction + -
Assignment =

Example:

int x = 2 + 3 * 4;   // 14, not 20
int y = (2 + 3) * 4; // 20

5.2 Mixed Expressions

If one operand is double, the result becomes double:

double result = 5 / 2;     // 2.0 (integer division first)
double result2 = 5.0 / 2;  // 2.5 (floating-point division)

6. Augmented Assignment and Increment/Decrement

6.1 Augmented Assignment

Shortcut operators:

x += 3;  // same as x = x + 3
y *= 2;  // same as y = y * 2

6.2 Increment and Decrement

++ increases by 1, -- decreases by 1. Two forms:

int a = 5;
System.out.println(a++); // prints 5, then a = 6
System.out.println(++a); // a = 7, then prints 7

Remember:

Postfix (a++) → use then increment
Prefix (++a) → increment then use

7. Common Mistakes to Avoid

Using = instead of == in comparisons.
Integer division when you expect decimals.
Forgetting to initialize a variable before use.
Losing data in type conversion (e.g., (int) 3.9 → 3).

8. Typical Exam Questions

Example 1:

What is the output of the following code?
int x = 5, y = 2;
double z = x / y;
System.out.println(z);

Answer: 2.0 (integer division before assignment)

Example 2:

Which operator has the highest precedence? A. + B. * C. = D. ++ Answer: D

Example 3:

What is the output of this code?

int a = 3;
int b = ++a + a++;
System.out.println(b);

Explanation: ++a makes a = 4, adds 4, then a++ uses 4 and increments to 5. So b = 8.

Would you like me to continue with Part III: Control Structures (Selection and Looping Statements) next? It covers if, switch, for, while, and is the core of procedural logic in the Java exam.

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