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Arithmetic expressions

• An expression 表达式 is a piece of code that computes a value.
• Java's arithmetic operators 运算符: +, -, *, /, and % (remainder).
• % is the modulus 取模 operator: 17 % 5 is 2 (the leftover after dividing).
• Expressions follow the usual order of operations (*, /, % before +, -).

Precedence and grouping

• Multiplication and division bind tighter than addition and subtraction.
• Use parentheses to force the order you want: (a + b) * c.
• Operators of equal precedence evaluate left to right.
• When in doubt, add parentheses — clarity beats cleverness.

print stays on the line, println ends it; + adds numbers until a String appears

Printing output

• System.out.println(x) prints x and moves to a new line.
• System.out.print(x) prints x with no new line after.
• The + operator concatenates 拼接 strings: "Score: " + score.
• Mixing a String with a number in + turns the number into text.

String concatenation vs. addition

• If either side of + is a String, + means concatenation, not addition.
• "" + 3 + 4 gives "34" (text), but 3 + 4 + "" gives "7" (added first).
• Evaluation is left to right, so the first + decides the mode.
• Wrap numeric math in parentheses when concatenating: "Sum: " + (3 + 4).