Declaring and assigning

A variable is a typed storage box: you declare its type once, and Java enforces it forever.

int score = 95;            // whole numbers
double average = 91.5;     // decimals
boolean passed = true;     // true or false
char letter = 'A';         // ONE character, single quotes
String name = "An";        // text, double quotes

Assignment replaces; it does not add. Trace this:

int x = 5;
x = 10;        // x is 10 now — the 5 is gone
x = x + 3;     // right side evaluated first: 10 + 3, then stored

After those three lines, x is 13. The right side of = is always evaluated before the store happens.

boolean holds exactly two values, true and false — no numbers, no "truthy" shortcuts like other languages. char uses single quotes ('A'), String uses double quotes ("A"). Mixing them is a classic compile error.