1.4 Assignment Statements and Input

What you need to know

• = is assignment: evaluate the right side, store it in the variable on the left. It is not equality (that's ==).
• The right side is evaluated using the variable's current value, so x = x + 1 increments. The expression must produce a value compatible with the variable's type.
• Assigning one primitive variable to another copies the value. Assigning one reference variable to another copies the reference — both then point to the same object.
• Input comes from Scanner. Create one with Scanner input = new Scanner(System.in); then read with nextInt(), nextDouble(), nextLine(), or next() (one word).
• hasNext(), hasNextInt(), hasNextLine() return whether more input is available — used to loop over input of unknown length.
• The nextInt/nextLine trap: nextInt() leaves the newline in the buffer, so a following nextLine() returns an empty string. The fix is an extra nextLine() to consume it.
• Swapping two variables requires a temporary: int temp = a; a = b; b = temp;

Worked example

Scanner in = new Scanner(System.in);
System.out.print("Age: ");
int age = in.nextInt();
in.nextLine();                 // consume leftover newline
System.out.print("Name: ");
String name = in.nextLine();
System.out.println(name + " is " + age);

Without line 4, if the user types 17 and Enter, name becomes the empty string because nextLine() reads the leftover newline. This exact trap is the reason Scanner questions exist.

Going deeper

The nuance, edge cases, and connections that turn a 3 into a 5.

• Assignment is right-to-left. Evaluate the entire right side using current values; then store in the left variable. x = x + 1 is not an equation; it's "compute x + 1, then overwrite x."
• Copying primitives copies values. After int b = a;, a and b are independent. Copying references copies addresses. After Dog b = a;, both point at one Dog. (Full treatment in 1.13.)
• Scanner reads tokens or lines. next() reads one whitespace-delimited token. nextLine() reads to the end of the line, including spaces. nextInt() / nextDouble() read one token and convert it — and throw InputMismatchException if it isn't a number.
• The buffer trap in detail: the user types 42⏎. nextInt() consumes 42 and leaves ⏎. The next nextLine() reads up to that ⏎ — an empty String — and consumes it. Only the following nextLine() gets the actual next line. Fix: call nextLine() once after nextInt() to discard the leftover.
• hasNext family: hasNext(), hasNextInt(), hasNextDouble(), hasNextLine() return booleans and don't consume anything. They're how you loop over input of unknown length — and they're the same methods used for files in 4.6.
• Scanner is in java.util — needs an import. Creating one: new Scanner(System.in) for keyboard.
• A swap needs a temporary variable. The three-line pattern is worth memorizing because it appears inside sorting algorithms in Unit 4.

Mistakes that cost points

• Using == for assignment or = for comparison. if (x = 5) won't compile (int isn't boolean). x == 5; compiles but does nothing.
• Forgetting the buffer clear after nextInt. The next nextLine() gets an empty string. This is the single most common Scanner bug and it's tested.
• Using next() when you need a whole line. next() stops at the first space. "Ram Singh" becomes "Ram".
• Two-line swap. a = b; b = a; leaves both equal to b's original value.

Practice questions

Written in the style of the real exam. Try each one before revealing the answer.

int a = 3;
int b = 7;
a = b;
b = a;