Language basics

Jumper reads like Java. The differences: types are optional, there are tables and functions as values, and a script is just statements from top to bottom - no class Main, no main method.

// a line comment
/* a block comment */
println("statements run from top to bottom");

Variables and types

dyn x = 1;          // dynamic: holds any value, can change type
x = "now a string";

int count = 0;      // typed: checked when the script is parsed and when it runs
long big = 9000000000L;
double ratio = 0.75;
boolean ready = true;
String name = "Jumper";

Type	Values	Default
dyn	anything	null
int, long, double	numbers	0
boolean	true, false	false
String	text or null	null
a class (script or Java)	an instance of it or null	null

A wrong value in a typed variable is an error. When the type is known before the script runs, it is reported right away:

int n = "five";   // Syntax error: Cannot assign string to int

Numbers widen (int to long to double), they never narrow by themselves. Convert explicitly with int(x), long(x), double(x) - there are no casts like (int) x.

Several variables in one declaration: int a = 1, b = 2;.

Numbers

int i = 42;
int hex = 0xFF;
int million = 1_000_000;
long l = 10L;
double d = 3.14e-2;

println(7 / 2);     // 3 - int division
println(7 / 2.0);   // 3.5
println(7 % 3);     // 1

Integer division by zero is an error, double division gives Infinity or NaN. Overflow wraps around as in Java.

Strings

"..." and '...' are the same: a string of any length (there is no char). Escapes: \n \t \r \0 \\ \" \' and \uXXXX.

String s = "Jumper";
println(s.length);          // 6
println(s.toUpperCase());   // JUMPER - any method of java.lang.String
println(s[0]);              // J - a one-letter string
println("a" + 1 + 2);       // a12

Truth

Only null and false are false. 0, "" and empty tables are true.

&& and || return one of their operands, so || gives a default value:

dyn nick = null;
println(nick || "guest");   // guest

Operators

arithmetic	+ - * / % (+ also joins strings)
comparison	== != < <= > >=
logic	&& || !
bits	& | ^ << >> >>>
assignment	= += -= *= /= %=, ++, --
conditional	cond ? a : b

== compares numbers by value (1 == 1L is true), strings and arrays by content, tables and objects by reference.

Control flow

int n = 7;

if (n > 5) {
    println("big");
} else {
    println("small");
}

while (n > 0) n--;

do {
    n++;
} while (n < 3);

for (int i = 0; i < 3; i++) {
    if (i == 1) continue;
    println(i);
}

for (dyn item : ["a", "b", "c"]) {   // for-each needs a type: dyn, int, String...
    println(item);
}

for-each walks arrays, Java collections and iterators, the keys of a table or a Java Map, and the characters of a string.

break and continue work in loops. There are no labels.

switch

Arrows only, no fall-through. Several values in one case:

int day = 6;

switch (day) {
    case 6, 7 -> println("weekend");
    default   -> println("weekday");
}

String kind = switch (day) {
    case 1  -> "start";
    case 6, 7 -> "rest";
    default -> "work";
};
println(kind);   // rest

A switch used as a value must have a default.

Errors

throw throws any value. catch has no type: it gets whatever was thrown.

try {
    throw { code: 404, reason: "not found" };
} catch (e) {
    println(e.code);          // 404
} finally {
    println("done");
}

try {
    error("something broke");  // a Jumper error with a message
} catch (e) {
    println(e.message);        // something broke
}

An exception from a Java call reaches catch as the Java exception itself, so isa(e, NumberFormatException) works.

Errors of the access policy and cancellation by the host cannot be caught by the script.

Scopes

• A block { } is a scope. Variables are visible after their declaration, until the end of the block.
• A function can be called above its declaration in the same block. A class cannot be used with new before its declaration has run.
• An inner block may declare a name that already exists outside (it hides the outer one); the same name twice in one block is an error.