College Board Unit 1
How it works
The + operator concatenates when at least one operand is a String, and adds when both operands are numeric. Java evaluates left to right, so the position of the first String in the expression decides whether earlier numbers are added or appended. Any primitive concatenated with a String is converted to its text form, and objects are converted using their toString method.
Read a mixed expression left to right, switching to concatenation from the first String onward.
Common error
An expression beginning with two ints adds them; the same operands after a String are appended as digits.
Worked example
Java
System.out.println(2 + 5 + " points");
System.out.println("points " + 2 + 5);Output
7 points
points 25The first line adds before it reaches a String. The second line concatenates from the first operation.
Exercises
1.4.1 · Trace
Determine the exact output. Explain the Java rule or state changes that produce it.
Java
System.out.println("A" + (3 + 4) + 2);Show solution
Output
A72The parenthesized sum becomes 7; the remaining operations concatenate.
1.4.2 · Write code
Given int correct = 8 and int bonus = 3, print exactly Score: 11. Make the numeric addition happen before concatenation.
Show solution
One valid solution is shown below. Other implementations satisfying the contract are also valid.
Java
int correct = 8;
int bonus = 3;
System.out.println("Score: " + (correct + bonus));