ExpressionTest.java

package customers;

import org.assertj.core.api.Assertions;
import org.junit.jupiter.api.Test;

import java.util.LinkedHashMap;
import java.util.LinkedList;
import java.util.List;
import java.util.Map;
import java.util.function.BiFunction;

class ExpressionTest extends Assertions {

    @Test
    void test() throws Exception {
        // Given
        final Expression expression = Expression.of("5 + 5");

        // When
        final double result = expression.evaluate();

        // Then
        assertThat(result).isEqualTo(10);
    }
}

class Expression {
    private final List<BiFunction<Double, Double, Double>> operations;

    Expression(
            final List<BiFunction<Double, Double, Double>> operations) {
        this.operations = operations;
    }

    static Expression of(final String exp) {
        List<BiFunction<Double, Double, Double>> operations = new LinkedList<>();
        final String[] split = exp.split("\\s");
        final int length = split.length;
        for (int i = 0; i <= length; i++) {
            final String s = split[i];
            if (s.matches("^(?:(?:-)?\\d+(?:\\.\\d+)?)$")) {
                operations.add((left, right) -> Double.parseDouble(s));
            } else {
                final BiFunction<Double, Double, Double> operation = Expression.of(s.charAt(0));
                operations.add(operation);
            }
        }
        return new Expression(operations);
    }

    public double evaluate() {
        return 0;
    }

    public List<BiFunction<Double, Double, Double>> getOperations() {
        return new LinkedList<>(operations);
    }

    static BiFunction<Double, Double, Double> of(
            char symbol) {
        final Map<Character, BiFunction<Double, Double, Double>> operations =
                new LinkedHashMap<>();
        operations.put('+', (left, right) -> left + right);
        operations.put('-', (left, right) -> left - right);
        operations.put('*', (left, right) -> left * right);
        operations.put('/', (left, right) -> left / right);
        return operations.get(symbol);
    }

}