Explicit and implicit

In short: Explicit means stated expressly, implicit means silently added by the system. The contrast shows up in type conversions, constructors, imports and interfaces.

In more detail: With explicit code you write down what should happen and the reader sees it. Implicit lets the language take work off your hands but can create surprises.

In detail

• Type conversion: implicit (automatic): double d = 5; (int becomes double). Explicit (cast): int i = (int) 5.7; (gives 5, the decimals are lost).
• Constructor: if a class has none, Java creates an implicit default constructor.
• Type inference: var x = 5; is an implicit type, int x = 5; an explicit one.
• JavaScript: implicit conversion ("5" * 2 is 10, "5" + 2 is "52") is a source of bugs, so prefer === and explicit conversion.
• Rule: where data loss threatens, the language usually demands an explicit cast.

See also: Constants, Char (character)