添加三个Integer变量将结果显示为Null，为什么？

Integer a = null;
Integer b = null;
Integer c = null;

// 尝试相加会导致NullPointerException
Integer result = a + b + c; // 抛出NullPointerException

Integer a = 0;
Integer b = 0;
Integer c = 0;
Integer result = a + b + c; // 结果为0

public Integer getValue() {
    return null;
}

Integer a = getValue();
Integer b = getValue();
Integer c = getValue();

Integer result = a + b + c; // 抛出NullPointerException

Integer a = getValue();
Integer b = getValue();
Integer c = getValue();

if (a != null && b != null && c != null) {
    Integer result = a + b + c;
} else {
    // 处理null情况
}

import java.util.Optional;

Optional<Integer> a = Optional.ofNullable(getValue());
Optional<Integer> b = Optional.ofNullable(getValue());
Optional<Integer> c = Optional.ofNullable(getValue());

Integer result = a.orElse(0) + b.orElse(0) + c.orElse(0);