Useful Java Patterns

This guide offers tutorials in Java and Kotlin, covering patterns like Java Stream, list comprehension, switch expressions, and iterating through lists with indices. Additionally, it discusses eclipse and JetBrains plugin support for BeanShell syntax highlighting. The guide also teaches various methods to count array element occurrences in both languages.

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java stream guide

learn java and kotlin the same time?

beanshell syntax highlight seems to be rare. beanshell workspace has the highlight.

use v2.1.1 and above for varargs support.

there are eclipse and jetbrains plugin support for beanshell.

creating lists

var a = new ArrayList<>(Arrays.asList(1,2,3));
var mset = new HashSet<>();
mset.addAll(a);
var mset2 = a.stream().collect(Collectors.toSet());
var mymin = Collections.min(a);
var mymax = Collections.max(a);
Collections.reverse(a);

var a = arrayOf(1,2,3);
print(a.contentToString())
print(a.max())
print(a.min())
a.reverse()

creating hashmap

var ah= new HashMap<>();
ah.put(1,2);

var ah = hashMapOf(1 to 2, 2 to 3)

iterate lists with indices

java double colon :: operator acts as anonymous function

var l = a.listIterator();
while (l.hasNext()){
var index = l.nextIndex();
var val = l.next();
System.out.println("INDEX: "+index+" ELEM: "+val);
}
IntStream.range(0,a.size()).forEach(index -> a.get(index));
var index=0;
for (int i: a){
index++;
}

a.forEachIndexed{ind, elem -> println("index? $ind"); println("elem? $elem")}
for (var i in a.indices){
var elem = a[i]
}
a.indices.forEach {
var elem = a[it]
elem
}

list comprehension

var mlist = a.stream().map(x-> x*2).collect(Collectors.toList());
var evenNums = a.stream().filter(x-> x%2 == 0).collect(Collectors.toList());
var mmap = a.stream().collect(Collectors.toMap(x->x.getId(),x->x.getName()));

switch expressions

var val = 2;
var mswitch = switch (val){
case 1,2,3 -> {
yield "good";
}
case 4,5,6 -> {
yield "bad";
} // either throw or yield.
default -> {
System.out.println("out of expectation");
yield "really bad";
}
};
System.out.println(mswitch);

var grade = 30
var res =  when(grade) {
in 0..40 -> "Fail"
in 41..70 -> "Pass"
in 71..100 -> "Distinction"
else -> false
}
print(res)
var mcase = 1
var res = when(mcase){
1 -> "good"
2 -> "bad"
else -> "really bad"
}
print(res)

count occurance of elements in array

String[] array = {"name1","name2","name3","name4", "name5", "name2"};
Arrays.stream(array)
.collect(Collectors.groupingBy(s -> s))
.forEach((k, v) -> System.out.println(k+" "+v.size()));
List asList = Arrays.asList(array);
Set<String> mySet = new HashSet<String>(asList);
for(String s: mySet){
System.out.println(s + " " + Collections.frequency(asList,s));
}
Map<String, Long> map = Arrays.stream(array)
.collect(Collectors.groupingBy(s -> s, Collectors.counting()));

var a = arrayOf(1,2,3,3,3,3)
a.toSet().forEach{it -> println("elem? $it"); println("count? "+a.count{it2->it2 == it})}

lambdas

Consumer mcons = (n) -> {System.out.println(n);}
mcons.andThen(mcons).accept("mval");
Function <Integer,Integer> mfunc = n-> n+1;
Supplier msup = () -> 1;
var mval = msup.get();

var mfunc = {n :Int -> n+1}
var mprint = {n:Any -> print(n)}