Task: Sum to 100
Go: package main

import (
	"fmt"
	"sort"
)

const pow3_8 = 3 * 3 * 3 * 3 * 3 * 3 * 3 * 3 
const pow3_9 = 3 * pow3_8                    
const maxExprs = 2 * pow3_8                  

type op uint8

const (
	Add  op = iota 
	Sub            
	Join           
)




type code uint16


func (c code) evaluate() (sum int) {
	num, pow := 0, 1
	for k := 9; k >= 1; k-- {
		num += pow * k
		switch op(c % 3) {
		case Add:
			sum += num
			num, pow = 0, 1
		case Sub:
			sum -= num
			num, pow = 0, 1
		case Join:
			pow *= 10
		}
		c /= 3
	}
	return sum
}

func (c code) String() string {
	buf := make([]byte, 0, 18)
	a, b := code(pow3_9), code(pow3_8)
	for k := 1; k <= 9; k++ {
		switch op((c % a) / b) {
		case Add:
			if k > 1 {
				buf = append(buf, '+')
			}
		case Sub:
			buf = append(buf, '-')
		}
		buf = append(buf, '0'+byte(k))
		a, b = b, b/3
	}
	return string(buf)
}

type sumCode struct {
	sum  int
	code code
}
type sumCodes []sumCode

type sumCount struct {
	sum   int
	count int
}
type sumCounts []sumCount


func (p sumCodes) Len() int            { return len(p) }
func (p sumCodes) Swap(i, j int)       { p[i], p[j] = p[j], p[i] }
func (p sumCodes) Less(i, j int) bool  { return p[i].sum < p[j].sum }
func (p sumCounts) Len() int           { return len(p) }
func (p sumCounts) Swap(i, j int)      { p[i], p[j] = p[j], p[i] }
func (p sumCounts) Less(i, j int) bool { return p[i].count > p[j].count }


func (sc sumCode) String() string {
	return fmt.Sprintf("% 10d = %v", sc.sum, sc.code)
}
func (sc sumCount) String() string {
	return fmt.Sprintf("% 10d has %d solutions", sc.sum, sc.count)
}

func main() {
	
	expressions := make(sumCodes, 0, maxExprs/2)
	counts := make(sumCounts, 0, 1715)
	for c := code(0); c < maxExprs; c++ {
		
		
		
		sum := c.evaluate()
		if sum >= 0 {
			expressions = append(expressions, sumCode{sum, c})
		}
	}
	sort.Sort(expressions)

	
	sc := sumCount{expressions[0].sum, 1}
	for _, e := range expressions[1:] {
		if e.sum == sc.sum {
			sc.count++
		} else {
			counts = append(counts, sc)
			sc = sumCount{e.sum, 1}
		}
	}
	counts = append(counts, sc)
	sort.Sort(counts)

	

	fmt.Println("All solutions that sum to 100:")
	i := sort.Search(len(expressions), func(i int) bool {
		return expressions[i].sum >= 100
	})
	for _, e := range expressions[i:] {
		if e.sum != 100 {
			break
		}
		fmt.Println(e)
	}

	fmt.Println("\nThe positive sum with maximum number of solutions:")
	fmt.Println(counts[0])

	fmt.Println("\nThe lowest positive number that can't be expressed:")
	s := 1
	for _, e := range expressions {
		if e.sum == s {
			s++
		} else if e.sum > s {
			fmt.Printf("% 10d\n", s)
			break
		}
	}

	fmt.Println("\nThe ten highest numbers that can be expressed:")
	for _, e := range expressions[len(expressions)-10:] {
		fmt.Println(e)
	}
}

Java: 
package rosettacode;

import java.io.PrintStream;
import java.util.Arrays;
import java.util.Collections;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Iterator;
import java.util.Map;
import java.util.Set;

public class SumTo100 implements Runnable {

    public static void main(String[] args) {
        new SumTo100().run();
    }

    void print(int givenSum) {
        Expression expression = new Expression();
        for (int i = 0; i < Expression.NUMBER_OF_EXPRESSIONS; i++, expression.next()) {
            if (expression.toInt() == givenSum) {
                expression.print();
            }
        }
    }

    void comment(String commentString) {
        System.out.println();
        System.out.println(commentString);
        System.out.println();
    }

    @Override
    public void run() {
        final Stat stat = new Stat();

        comment("Show all solutions that sum to 100");
        final int givenSum = 100;
        print(givenSum);

        comment("Show the sum that has the maximum number of solutions");
        final int maxCount = Collections.max(stat.sumCount.keySet());
        int maxSum;
        Iterator<Integer> it = stat.sumCount.get(maxCount).iterator();
        do {
            maxSum = it.next();
        } while (maxSum < 0);
        System.out.println(maxSum + " has " + maxCount + " solutions");

        comment("Show the lowest positive number that can't be expressed");
        int value = 0;
        while (stat.countSum.containsKey(value)) {
            value++;
        }
        System.out.println(value);

        comment("Show the ten highest numbers that can be expressed");
        final int n = stat.countSum.keySet().size();
        final Integer[] sums = stat.countSum.keySet().toArray(new Integer[n]);
        Arrays.sort(sums);
        for (int i = n - 1; i >= n - 10; i--) {
            print(sums[i]);
        }
    }

    private static class Expression {

        private final static int NUMBER_OF_DIGITS = 9;
        private final static byte ADD = 0;
        private final static byte SUB = 1;
        private final static byte JOIN = 2;

        final byte[] code = new byte[NUMBER_OF_DIGITS];
        final static int NUMBER_OF_EXPRESSIONS = 2 * 3 * 3 * 3 * 3 * 3 * 3 * 3 * 3;

        Expression next() {
            for (int i = 0; i < NUMBER_OF_DIGITS; i++) {
                if (++code[i] > JOIN) {
                    code[i] = ADD;
                } else {
                    break;
                }
            }
            return this;
        }

        int toInt() {
            int value = 0;
            int number = 0;
            int sign = (+1);
            for (int digit = 1; digit <= 9; digit++) {
                switch (code[NUMBER_OF_DIGITS - digit]) {
                    case ADD:
                        value += sign * number;
                        number = digit;
                        sign = (+1);
                        break;
                    case SUB:
                        value += sign * number;
                        number = digit;
                        sign = (-1);
                        break;
                    case JOIN:
                        number = 10 * number + digit;
                        break;
                }
            }
            return value + sign * number;
        }

        @Override
        public String toString() {
            StringBuilder s = new StringBuilder(2 * NUMBER_OF_DIGITS + 1);
            for (int digit = 1; digit <= NUMBER_OF_DIGITS; digit++) {
                switch (code[NUMBER_OF_DIGITS - digit]) {
                    case ADD:
                        if (digit > 1) {
                            s.append('+');
                        }
                        break;
                    case SUB:
                        s.append('-');
                        break;
                }
                s.append(digit);
            }
            return s.toString();
        }

        void print() {
            print(System.out);
        }

        void print(PrintStream printStream) {
            printStream.format("%9d", this.toInt());
            printStream.println(" = " + this);
        }
    }

    private static class Stat {

        final Map<Integer, Integer> countSum = new HashMap<>();
        final Map<Integer, Set<Integer>> sumCount = new HashMap<>();

        Stat() {
            Expression expression = new Expression();
            for (int i = 0; i < Expression.NUMBER_OF_EXPRESSIONS; i++, expression.next()) {
                int sum = expression.toInt();
                countSum.put(sum, countSum.getOrDefault(sum, 0) + 1);
            }
            for (Map.Entry<Integer, Integer> entry : countSum.entrySet()) {
                Set<Integer> set;
                if (sumCount.containsKey(entry.getValue())) {
                    set = sumCount.get(entry.getValue());
                } else {
                    set = new HashSet<>();
                }
                set.add(entry.getKey());
                sumCount.put(entry.getValue(), set);
            }
        }
    }
}