Foo.java

// INPUT
final Map<ItemStack, Double> myItemsWithWeight; // Initialized somewhere before that

// Data
final List<ItemStack> myItems = new ArrayList<>(myItemsWithWeight.size());
final double[] risingWeights = new double[myItemsWithWeight.size()];

// Compute total weight
double total = 0;
for (final double d : myItemsWithWeight.values()) {
  total += d;
}

// Transform input into useful data
int i = 0;
double previous = 0;
for (final Entry<ItemStack, Double> e : myItemsWithWeight.entrySet()) {
  myItems.set(i, e.getKey());
  previous += e.getValue() / total;
  risingWeights[i] = previous;
  i++;
}

// Now use our wonderful data
final float f = new Random().nextFloat();
for (i = 0; i < risingWeights.length; i++) {
  if (f < risingWeights[i]) {
    return myItems.get(i); // Enjoy
  }
}