Groovy Cheatsheet

I use Groovy as a scripting language, to help me do quick and dirty processing. I often need to reference its syntax. So here’s my personal cheat sheet.

Basic

Prints text to the console. For complex GString expressions, use ${expression}.

value = 6
println "The value is $value."

Loops

Loop a range (both inclusive).

for ( i in 1..10 ) {
	println i
}

Loop a range (upper exclusive).

for ( i in 0..<10 ) {
	println i
}

Loop a list.

for ( i in [2, 3, 5, 7, 11] ) {
	println i
}

Loop a map.

for ( entry in [a:1, b:2, c:3] ) {
	println entry.value
}

Closure style looping. The collection being iterated is returned.

[2, 3, 5, 7, 11].each { println it }
[2, 3, 5, 7, 11].each { prime -> println prime }
[2, 3, 5].eachWithIndex { p, i -> println "$i: $p" }
[a:1, b:2, c:3].each { println "key: ${it.key}, value: ${it.value}" }
[a:1, b:2, c:3].each { k, v -> println "key: $k, value: $v" }

Strings

Substrings

text = "Hello World!"
text[0]	// Returns "H"
text[0..4]	// Returns "Hello"
text[0..<4]	// Returns "Hell"
text[-1]	// Returns "!" (last char)
text[-6..-1]	// Returns "World!" (last 6th to last char)

Tokenizing. Returns ["Hello", "World!"]. Note that using split(” “) in this case (where there are 2 spaces between Hello and World!) will result in 3 elements ["Hello", "", "World!"].

"Hello  World!".tokenize(" ")

Regex

"Hello World!" ==~ /.*World.*/	// true

matcher = ( "email@address.com" =~ /(\w+)@([\w\.]+)/ )
matcher[0][1]    // Returns "email"
matcher[0][2]    // Returns "address.com"

Files

Iterate each file in a directory. it is a File object.

new File("d:/").eachFile { println it }

Match certain files in directory. See regular expression.

new File("d:/").eachFileMatch(~/.*\.java/) { println it }

Iterate all files in directory recursively.

new File("d:/").eachFileRecurse { println it }

Read whole text file.

new File("test.txt").getText()

Read text file line by line.

new File("test.txt").eachLine { println it }

Write text to file.

new File("test.txt").withWriter { out-> out.println("Hello World!") }

Collections

Basic operations

list = []	// new empty list
list = [2, 3, 5, 7, 11]	// new initialized list
list[3]	// Returns 7.
list[-1]	// Returns 11 (last item)
list[1..2]	// Returns [3, 5] (sublist)
[2, 3] << 5	// append 5 to list. Returns [2, 3, 5]
[2, 3] + 5	// append. same as above.
[2, 3, 5, 7] - 3	// remove 3 from list. Returns [2, 5, 7]
list.remove(1)	// remove item 1 (integer 3) from list. Removed item is returned.

map = [:]
map = [a:1, b:2, c:3]
map["b"]	// returns 2
map.c	// returns 3
map["b"] = 5	// map.put("b", 5")

Find first item matching closure. Returns 7.

[2, 3, 5, 7, 11].find { it > 6 }

Find all items matching closure. Returns [7, 11].

[2, 3, 5, 7, 11].findAll { it > 6 }

Operate on each element. Returns [3, 4, 6, 8].

[2, 3, 5, 7].collect { it+1 }

Join elements into String. Returns “2 3 5 7″.

[2, 3, 5, 7].join(" ")

Date/Time

Basic operations

d = new Date()	// Today
++d	// Tomorrow (d++ behaves consistently like primitives)
--d	// Yesterday (d-- behaves consistently like primitives)
d+=2	// 2 days later
d.format("dd-MM-yyyy")	// date formatting

XML
A section I feel will be useful. Will be populated once I’ve used it.

SQL
A section I feel will be useful. Will be populated once I’ve used it.

Threading

Run a thread.

thread = Thread.start { doSomething(); }

Networking

Grab content from URL. Operations are identical to stream manipulation.

new URL("http://www.google.com/").getText()

Connect via TCP.

new Socket("host", 23).withStreams { inn, out -> }

Misc

Object inspection

class Man {
    def name
    def age
    
    Man(name, age) {
        this.name = name;
        this.age = age;   
    }
}
john = new Man("John", 40)
john.dump()

Shorthand for Math.pow().

2**3	// Returns 8.

Use ?. notation to help get around NPEs.

[1, 2, null, 4].each { println it?.class }

GUI – SwingBuilder
Below is an copy-style example of what can be done. Refer to the Widgets List for more documentation. Most bean-style attributes can be directly set in the constructor.

import java.awt.*;
import javax.swing.*;
import java.awt.BorderLayout as BL;
swing = new groovy.swing.SwingBuilder();

frame = swing.frame(title:'Frame', defaultCloseOperation:JFrame.DISPOSE_ON_CLOSE, bounds:[100, 100, 800, 600], visible:true) {
    lookAndFeel("system")
    borderLayout()
    panel(constraints:BL.NORTH) {
        flowLayout(alignment:FlowLayout.LEFT)
        label(text:"Country: ")
        countryCombo = comboBox(items:["Japan", "China"], selectedIndex:0, actionPerformed: { changeValues(); } )
        label(text:"Area: ")
        areaCombo = comboBox(items:areas, selectedIndex:0, actionPerformed: { changeValues(); } )
    }
    plate = panel(constraints:BL.CENTER) {
        gridLayout(rows:10, cols:20)
        200.times {
            label(text:String.valueOf(it+1), horizontalAlignment:JLabel.CENTER, border:lineBorder(color:Color.LIGHT_GRAY), opaque:true)
        }
    }
}

From here you can see how to create JFrames, BorderLayout, GridLayout, JPanel, JLabel, JComboBox, Borders, ActionListeners, and how to set their attributes.