class MyMap extends java.util.LinkedHashMap {
    public final String foobar

    MyMap(Map map) {
        super(map)
        this.foobar = 'foobar'
    }

    def myFooBar() {
        return this.foobar
    }
}

node {
    def m = new MyMap([foo: 'bar', bar: 'foo'])
    print "bar=${m.bar}"
    print "foo=${m.foo}"
    print "foobar=${m.foobar}"
    print "foobar=${m.myFooBar()}"
} 

[Pipeline] node
Running on Jenkins in /build/workspace
[Pipeline] {
[Pipeline] echo
bar=foo
[Pipeline] echo
foo=bar
[Pipeline] echo
foobar=null
[Pipeline] echo
foobar=null
[Pipeline] }
[Pipeline] // node
[Pipeline] End of Pipeline
Finished: SUCCESS