Island perimeter code challenge in Java
Instead of looking for cool new features of Java i would strongly suggest that you concentrate on the basics of Java: Objects:

Think in Objects:

So far you have identified these Objects: Map, a Cell and an Island - so make some Objects of that type.

Map map = new Map(int[][] src);
Island island = map.extractIsland();
int diameter = island.getPerimater();


the Map needs some methods to properly extract the island

Cell cell = getCellAt(int x, int y);


and a method to gain relationship between each Cells on the map:

List&lt;Cell&gt; getNeighbours(Cell center);


and some elementary methods on the Cell class

boolean isWater();
boolean isLand();
int getAmountCoasts();


if you would have these elementary Objects you can simply create readable Code - see this example:

Cell center; //yes, here center.isLand() = true
List&lt;Cell&gt; neigbour = map.getNeighbours(center);
for (Cell neigbour: neigbours){
    if (neigbour.isWater()){
        center.addCoast();
    }
}


once you are here you can use features from java 8

List&lt;Cell&gt; landCells = cells.stream.filter(Cell::isLand).collect(CollectorsToList());
int perimeter = landCells.stream.mapToInt(Cell::.getAmountCoasts).sum();


Hint: you better let the Island class handle that, see first hints - Responsibility for coastLine is within the Island class!)

NOTE:

it's hard to think in objects whenever you try minimal code snippets as suggested from leet code. if minimal code is required, look maybe you are interested at CodeGolf...