Task: Numerical integration
Go: package main

import (
    "fmt"
    "math"
)


type spec struct {
    lower, upper float64               
    n            int                   
    exact        float64               
    fs           string                
    f            func(float64) float64 
}


var data = []spec{
    spec{0, 1, 100, .25, "x^3", func(x float64) float64 { return x * x * x }},
    spec{1, 100, 1000, float64(math.Log(100)), "1/x",
        func(x float64) float64 { return 1 / x }},
    spec{0, 5000, 5e5, 12.5e6, "x", func(x float64) float64 { return x }},
    spec{0, 6000, 6e6, 18e6, "x", func(x float64) float64 { return x }},
}


type method struct {
    name      string
    integrate func(spec) float64
}


var methods = []method{
    method{"Rectangular (left)    ", rectLeft},
    method{"Rectangular (right)   ", rectRight},
    method{"Rectangular (midpoint)", rectMid},
    method{"Trapezium             ", trap},
    method{"Simpson's             ", simpson},
}

func rectLeft(t spec) float64 {
    var a adder
    r := t.upper - t.lower
    nf := float64(t.n)
    x0 := t.lower
    for i := 0; i < t.n; i++ {
        x1 := t.lower + float64(i+1)*r/nf
        
        
        a.add(t.f(x0) * (x1 - x0))
        x0 = x1
    }
    return a.total()
}

func rectRight(t spec) float64 {
    var a adder
    r := t.upper - t.lower
    nf := float64(t.n)
    x0 := t.lower
    for i := 0; i < t.n; i++ {
        x1 := t.lower + float64(i+1)*r/nf
        a.add(t.f(x1) * (x1 - x0))
        x0 = x1
    }
    return a.total()
}

func rectMid(t spec) float64 {
    var a adder
    r := t.upper - t.lower
    nf := float64(t.n)
    
    
    
    
    
    
    x0 := t.lower - .5*r/nf
    for i := 0; i < t.n; i++ {
        x1 := t.lower + (float64(i)+.5)*r/nf
        a.add(t.f(x1) * (x1 - x0))
        x0 = x1
    }
    return a.total()
}

func trap(t spec) float64 {
    var a adder
    r := t.upper - t.lower
    nf := float64(t.n)
    x0 := t.lower
    f0 := t.f(x0)
    for i := 0; i < t.n; i++ {
        x1 := t.lower + float64(i+1)*r/nf
        f1 := t.f(x1)
        a.add((f0 + f1) * .5 * (x1 - x0))
        x0, f0 = x1, f1
    }
    return a.total()
}

func simpson(t spec) float64 {
    var a adder
    r := t.upper - t.lower
    nf := float64(t.n)
    
    
    dx0 := r / nf
    a.add(t.f(t.lower) * dx0)
    a.add(t.f(t.lower+dx0*.5) * dx0 * 4)
    x0 := t.lower + dx0
    for i := 1; i < t.n; i++ {
        x1 := t.lower + float64(i+1)*r/nf
        xmid := (x0 + x1) * .5
        dx := x1 - x0
        a.add(t.f(x0) * dx * 2)
        a.add(t.f(xmid) * dx * 4)
        x0 = x1
    }
    a.add(t.f(t.upper) * dx0)
    return a.total() / 6
}

func sum(v []float64) float64 {
    var a adder
    for _, e := range v {
        a.add(e)
    }
    return a.total()
}

type adder struct {
    sum, e float64
}

func (a *adder) total() float64 {
    return a.sum + a.e
}

func (a *adder) add(x float64) {
    sum := a.sum + x
    e := sum - a.sum
    a.e += a.sum - (sum - e) + (x - e)
    a.sum = sum
}

func main() {
    for _, t := range data {
        fmt.Println("Test case: f(x) =", t.fs)
        fmt.Println("Integration from", t.lower, "to", t.upper,
            "in", t.n, "parts")
        fmt.Printf("Exact result            %.7e     Error\n", t.exact)
        for _, m := range methods {
            a := m.integrate(t)
            e := a - t.exact
            if e < 0 {
                e = -e
            }
            fmt.Printf("%s  %.7e  %.7e\n", m.name, a, e)
        }
        fmt.Println("")
    }
}

Java: class NumericalIntegration
{

  interface FPFunction
  {
    double eval(double n);
  }
  
  public static double rectangularLeft(double a, double b, int n, FPFunction f)
  {
    return rectangular(a, b, n, f, 0);
  }
  
  public static double rectangularMidpoint(double a, double b, int n, FPFunction f)
  {
    return rectangular(a, b, n, f, 1);
  }
  
  public static double rectangularRight(double a, double b, int n, FPFunction f)
  {
    return rectangular(a, b, n, f, 2);
  }
  
  public static double trapezium(double a, double b, int n, FPFunction f)
  {
    double range = checkParamsGetRange(a, b, n);
    double nFloat = (double)n;
    double sum = 0.0;
    for (int i = 1; i < n; i++)
    {
      double x = a + range * (double)i / nFloat;
      sum += f.eval(x);
    }
    sum += (f.eval(a) + f.eval(b)) / 2.0;
    return sum * range / nFloat;
  }
  
  public static double simpsons(double a, double b, int n, FPFunction f)
  {
    double range = checkParamsGetRange(a, b, n);
    double nFloat = (double)n;
    double sum1 = f.eval(a + range / (nFloat * 2.0));
    double sum2 = 0.0;
    for (int i = 1; i < n; i++)
    {
      double x1 = a + range * ((double)i + 0.5) / nFloat;
      sum1 += f.eval(x1);
      double x2 = a + range * (double)i / nFloat;
      sum2 += f.eval(x2);
    }
    return (f.eval(a) + f.eval(b) + sum1 * 4.0 + sum2 * 2.0) * range / (nFloat * 6.0);
  }
  
  private static double rectangular(double a, double b, int n, FPFunction f, int mode)
  {
    double range = checkParamsGetRange(a, b, n);
    double modeOffset = (double)mode / 2.0;
    double nFloat = (double)n;
    double sum = 0.0;
    for (int i = 0; i < n; i++)
    {
      double x = a + range * ((double)i + modeOffset) / nFloat;
      sum += f.eval(x);
    }
    return sum * range / nFloat;
  }
  
  private static double checkParamsGetRange(double a, double b, int n)
  {
    if (n <= 0)
      throw new IllegalArgumentException("Invalid value of n");
    double range = b - a;
    if (range <= 0)
      throw new IllegalArgumentException("Invalid range");
    return range;
  }
  
  
  private static void testFunction(String fname, double a, double b, int n, FPFunction f)
  {
    System.out.println("Testing function \"" + fname + "\", a=" + a + ", b=" + b + ", n=" + n);
    System.out.println("rectangularLeft: " + rectangularLeft(a, b, n, f));
    System.out.println("rectangularMidpoint: " + rectangularMidpoint(a, b, n, f));
    System.out.println("rectangularRight: " + rectangularRight(a, b, n, f));
    System.out.println("trapezium: " + trapezium(a, b, n, f));
    System.out.println("simpsons: " + simpsons(a, b, n, f));
    System.out.println();
    return;
  }
  
  public static void main(String[] args)
  {
    testFunction("x^3", 0.0, 1.0, 100, new FPFunction() {
        public double eval(double n) {
          return n * n * n;
        }
      }
    );
    
    testFunction("1/x", 1.0, 100.0, 1000, new FPFunction() {
        public double eval(double n) {
          return 1.0 / n;
        }
      }
    );
    
    testFunction("x", 0.0, 5000.0, 5000000, new FPFunction() {
        public double eval(double n) {
          return n;
        }
      }
    );
    
    testFunction("x", 0.0, 6000.0, 6000000, new FPFunction() {
        public double eval(double n) {
          return n;
        }
      }
    );
    
    return;
  }
}