winterbe
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diff --git a/‎src/main/java/com/winterbe/java8/samples/lambda/InterfaceUsage.java‎
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+package com.winterbe.java8.samples.lambda;
+
+/**
+ * @author Benjamin Winterberg
+ */
+public class InterfaceUsage {
+
+	public static void main(String[] args) {
+		Formula formula1 = new Formula() {
+			@Override
+			public double calculate(int a) {
+				return sqrt(a * 100);
+			}
+		};
+
+		double result1 = formula1.calculate(100); // 100.0
+		double result2 = formula1.sqrt(-23); // 0.0
+		int result3 = Formula.positive(-4); // 0.0
+
+		// Formula formula2 = (a) -> sqrt(a * 100);
+
+		System.out.println(result1);
+		System.out.println(result2);
+		System.out.println(result3);
+	}
+}
+
+/**
+ * can define function in a interface class; function can be static.
+ */
+interface Formula {
+
+	double calculate(int a);
+
+	default double sqrt(int a) {
+		return Math.sqrt(positive(a));
+	}
+
+	static int positive(int a) {
+		return a > 0 ? a : 0;
+	}
+}