Method Analysis Examples

Stream

优化之前：

import java.util.List;

public class HelloWorld {
  public void test(List<String> list) {
    // ''stream().forEach()'' can be replaced with 'forEach()'' (may change semantics) 
    list.stream().forEach(System.out::println);
  }
}

优化之后：

import java.util.List;

public class HelloWorld {
    public void test(List<String> list) {
        list.forEach(System.out::println);
    }
}

判断逻辑：

void visitMethodCall(Call call) {
    Method m = call.resolve();
    if(m.equals(STREAM_FOR_EACH) {
        Expression qualifier = call.getQualifier();
        if(qualifier instanceof Call) {
            Method m1 = ((Call)qualifier).resolve();
            if(m1.equals(COLLECTION_STREAM) {
                reportWarning(call, "stream().forEach() can be replaced with forEach()");
            }
        }
    }
}

Constant conditions & exceptions warning

public class HelloWorld {
    public void test(String s, Object obj) {
        if (s == null) {
            // Method invocation 'trim' will produce 'NullPointerException'
            System.out.println(s.trim());
        }

        if (s.length() == 2) {
            // The call to 'substring' always fails as index is out of bounds
            System.out.println(s.substring(3));
        }

        if (obj instanceof Number) {
            // Casting 'obj' to 'String' will produce 'ClassCastException' for any non-null value
            System.out.println(((String) obj).isEmpty());
        }

        // Condition 's.equals("no")' is always 'false'
        // 因为如果s的值为no，那么它的length就应该是2。
        if (s.equals("no")) {
            System.out.println("Impossible!");
        }
    }
}

Condition is always true/false

public class HelloWorld {
    public void test(int x) {
        if (x < 0) {
            if (x > 1) {
                // Condition 'x > 1' is always 'false'
                System.out.println(x);
            }
        }
    }
}

public class HelloWorld {
    public void test(int idx) {
        if (Character.isLowerCase("Java".charAt(idx)) || idx < 4) {
            // Condition 'Character.isLowerCase("Java".charAt(idx)) || idx < 4' is always 'true'
            System.out.println(idx);
        }
    }
}

String.charAt(int idx):
- idx >= this.length -> fail;
- idx < 0 -> fail
第一种情况，如果idx == 0，那么false || true。
第二种情况，如果idx == 1，那么true || true。
第三种情况，如果idx == 2，那么true || true。
第四种情况，如果idx == 3，那么true || true。
第五种情况，如果idx > 3，那么StringIndexOutOfBoundsException || true。
第六种情况，如果idx < 0，那么StringIndexOutOfBoundsException || true。

public class HelloWorld {
    public void test() {
        if (5 > 5) {
            // Condition '5 > 5' is always 'false'
        }
    }
}

         ┌──────────────┐
         │ if statement │
         └──────────────┘
           ╱            ╲
        ┌───┐         ┌─────────┐
        │ > │         │ if body │
        └───┘         └─────────┘
        ╱   ╲             │
     ┌───┐ ┌───┐       ┌─────┐
     │ 5 │ │ 5 │       │ ... │
     └───┘ └───┘       └─────┘      

public class HelloWorld {
    public void test() {
        if (12 * 3 - 19 / 6 % 4 * Math.abs(-3) > 2) {
            // Condition '12 * 3 - 19 / 6 % 4 * Math.abs(-3) > 2' is always 'true'
        }
    }
}

public class HelloWorld {
    public void test(int x, int y) {
        if (x % 5 > y % 10 + 20) {
            // Condition 'x % 5 > y % 10 + 20' is always 'false'
        }
    }
}

x % 5 = [-4,4]
y % 10 + 20 = [-9, 9] + 20 = [11, 29]

public class HelloWorld {
    public void test(int x, int y) {
        if (x * 2 == y * 4 + 1) {
            // Condition 'x * 2 == y * 4 + 1' is always 'false'
        }
    }
}

x * 2 = even（偶数）
y * 4 + 1 = odd（奇数）
选择x * 2，按下Ctrl + Shift + P（Expression type），显示int类型，按下Ctrl + Shift + P twice (Advanced expression type)，显示Type: int, Range:even。
选择y * 4 + 1，按下Ctrl + Shift + P（Expression type），显示int类型，按下Ctrl + Shift + P twice (Advanced expression type)，显示Type: int, Range:<1> mod 4。

public class HelloWorld {
    public void test() {
        int x = 3;
        int y = x * x; // 9
        int z = x * 2; // 6
        if (x + z > y) { // 3 + 6 > 9
            // Condition 'x + z > y' is always 'false'
        }
    }
}

public class HelloWorld {
    public void test(int a, int b, int c, int d, int e, int f, int g, int h) {
        if (a > 0) {
            System.out.println("a > 0");
        }
        // a>0; a<=0
        if (b > 0) {
            System.out.println("b > 0");
        }
        // a>0, b>0; a>0, b<=0; a<=0, b>0; a<=0, b<=0
        if (c > 0) {
            System.out.println("c > 0");
        }
        // 8 states
        if (d > 0) {
            System.out.println("d > 0");
        }
        // 16 states
        if (e > 0) {
            System.out.println("e > 0");
        }
        // 32 states
        if (f > 0) {
            System.out.println("f > 0");
        }
        // 64 states
        if (g > 0) {
            System.out.println("g > 0");
        }
        // 128 states
        if (h > 0) {
            System.out.println("h > 0");
        }
        // 256 states
    }
}

public class HelloWorld {
    public void test(int x, int y) {
        int a = x > 0 ? 1 : 0;
        int b = y > 0 ? 1 : 0;
        if (a + b == 2) {
            if (x < 0) {
                // Condition 'x < 0' is always 'false'
            }
        }
    }
}

public class HelloWorld {
    public void test(int x, int y) {
        int a = x > 0 ? 1 : 0;
        // State#1: x > 0, a = 1
        // State#2: x <= 0, a = 0
        int b = y > 0 ? 1 : 0;
        // State#1: x > 0, a = 1, y > 0, b = 1
        // State#2: x > 0, a = 1, y <= 0, b = 0
        // State#3: x <= 0, a = 0, y > 0, b = 1
        // State#4: x <= 0, a = 0, y <= 0, b = 0
        if (a + b == 2) {
            // State#1: x > 0, a = 1, y > 0, b = 1
            if (x < 0) {
                // Condition 'x < 0' is always 'false'
            }
        }
    }
}

public void test(int x, int y) {
    int a = x > 0 ? 1 : 0;
    int b = y > 0 ? 1 : 0;
    if (a + b == 1) {
        if (x == y) {
            // Condition 'x == y' is always 'false'
        }
    }
}

public class HelloWorld {
    public void test(int x, int y) {
        int a = x > 0 ? 1 : 0;
        // State#1: x > 0, a = 1
        // State#2: x <= 0, a = 0
        int b = y > 0 ? 1 : 0;
        // State#1: x > 0, a = 1, y > 0, b = 1
        // State#2: x > 0, a = 1, y <= 0, b = 0
        // State#3: x <= 0, a = 0, y > 0, b = 1
        // State#4: x <= 0, a = 0, y <= 0, b = 0
        if (a + b == 1) {
            // State#2: x > 0, a = 1, y <= 0, b = 0
            // State#3: x <= 0, a = 0, y > 0, b = 1
            if (x == y) {
                // Condition 'x == y' is always 'false'
            }
        }
    }
}

Type constrains

public class HelloWorld {
    public void test(Object obj) {
        if (obj instanceof Number) return;
        // State: obj not instanceof Number
        if (obj instanceof CharSequence) return;
        // State not instanceof Number, CharSequence
        if (obj instanceof Integer) {
            // Condition 'obj instanceof Integer' is always 'false'
            System.out.println("Impossible");
        }
        if (obj instanceof StringBuilder) {
            // Condition 'obj instanceof StringBuilder' is always 'false'
            System.out.println("Also impossible");
        }
    }
}

public class HelloWorld {
    static class A {
    }

    static class B extends A {
    }

    public void test(A a1) {
        // a1 instanceof A
        B b1 = (B) a1;
        A a2 = new A();
        // a2 exactly A
        // Casting 'a2' to 'B' will produce 'ClassCastException' for any non-null value
        B b2 = (B) a2;
    }
}

Nullness

public class HelloWorld {
    public void test(String s) {
        if (s == null) {
            System.out.println("String is null!");
        }

        System.out.println(s.trim());
    }
}

public void test(String s) {
    // 没有提示
    System.out.println(s.trim());
}

Nullness kinds:

not-null
unknown
nullable
null

public class HelloWorld {
    public void test(String s) {
        // State: s:nullness = unknown
        if (s == null) {
            // State: s:nullness = null
            System.out.println("String is null!");
        }

        // State#1: s:nullness = not-null
        // State#2: s:nullness = null
        // Merged state: s:nullness = nullable
        // Method invocation 'trim' may produce 'NullPointerException'
        System.out.println(s.trim());
    }
}

Tracking relations

public class HelloWorld {
    public void test(Object a, Object b, Object c) {
        if (a == b && b == c) {
            if (a != c) {
                // Condition 'a != c' is always 'false'
            }
        }
    }
}

public class HelloWorld {
    public void test(String a, String b, String c) {
        if (a.equals(b) && b.equals(c)) {
            if (!a.equals(c)) {
                // Condition '!a.equals(c)' is always 'false'
            }
        }
    }
}

public class HelloWorld {
    public void test(int x, int y, int z) {
        if (x < y) return;
        if (y < z) return;
        if (x < z) {
            // Condition 'x < z' is always 'false'
        }
    }
}

public class HelloWorld {
    public void test(int x, int y) {
        if (x == y) return;
        int diff = x - y;
        if (diff == 0) {
            // Condition 'diff == 0' is always 'false'
        }
    }
}

Field

static class Point {
    int x;
    int y;
}

public void test(Point p) {
    p.x = 10;
    p.y = 10;
    // System.out.println("Hello!");
    if (p.x == p.y) { // Condition 'p.x == p.y' is always 'true'
        System.out.println("Impossible");
    }
}

import java.io.PrintStream;

public class HelloWorld {
    static class Point {
        int x;
        int y;
    }

    public static void test(Point p) {
        p.x = 10;
        p.y = 10;
        System.out.println("Hello!");
        if (p.x == p.y) { // Not always false anymore?
            System.out.println("Impossible");
        }
    }

    public static void main(String[] args) {
        Point p = new Point();
        System.setOut(new PrintStream(System.out) {
            @Override
            public void println(String x) {
                super.println(x);
                p.x = p.y = 0;
            }
        });
        test(p);
    }
}

Pure methods

public class HelloWorld {
    static class Point {
        int x;
        int y;
    }

    public static void test(Point p) {
        p.x = 10;
        p.y = 20;
        double z = Math.sqrt(2);
        if (p.x == p.y) { // Condition 'p.x == p.y' is always 'false'
            System.out.println("Impossible");
        }
    }
}

Locality tracking

public class HelloWorld {
    static class Point {
        int x;
        int y;
    }

    public static void test() {
        Point p = new Point();
        p.x = 10;
        p.y = 20;
        System.out.println("Hello!");
        if (p.x == p.y) { // Condition 'p.x == p.y' is always 'false'
            System.out.println("Impossible");
        }
    }
}

Array elements

public class HelloWorld {
    public static void test(int[] array) {
        if (array[0] == array[1]) {
            // Result of 'array[0] - array[1]' is always '0'
            int diff = array[0] - array[1];
            System.out.println(diff);
        }
    }
}

public class HelloWorld {
    public static void test(int[] array, int i) {
        if (array[0] == array[1]) {
            array[i] = 10;
            int diff = array[0] - array[1];
            System.out.println(diff);
        }
    }
}

public class HelloWorld {
    public static void test(int[] array, int i) {
        if (array[0] == array[1] && i >= 2) {
            array[i] = 10;
            int diff = array[0] - array[1];
            System.out.println(diff);
        }
    }
}

public class HelloWorld {
    public static void test(int[] array, int i, int j) {
        if (array[i] == array[j]) {
            int diff = array[i] - array[j];
            System.out.println(diff);
        }
    }
}

Getters

public class HelloWorld {
    public final class Point {
        private final int x;
        private final int y;

        public Point(int x, int y) {
            this.x = x;
            this.y = y;
        }

        public int getX() {
            return x;
        }

        public int getY() {
            return y;
        }
    }

    public static void test(Point p) {
        if (p.getX() % 2 == 1) {
            if (p.getX() == p.getY() * 2) {
                // Condition 'p.getX() == p.getY() * 2' is always 'false'
            }
        }
    }
}

Special fields

Array length
String length
Collection size(mutable)
Map size(mutable)
Optional value
Boxed value

Hardcoded contracts

String.isEmpty(): this.length == 0 -> true; else -> false

public class HelloWorld {
    public static void test(String s) {
        if (s.isEmpty()) return;
        int length = s.length();
        if (length >= 1) {
            // Condition 'length >= 1' is always 'true'
        }
    }
}

String.startsWith(String str): this.length < str.length -> false

public class HelloWorld {
    public static void test(String s) {
        if (s.startsWith("--")) {
            if (s.isEmpty()) {
                // Condition 's.isEmpty()' is always 'false'
            }
        }
    }
}

Method handlers

Method ranges

public class HelloWorld {
    public void test(LocalDate date) {
        int day = date.getDayOfMonth();
        if (day == 0) {
            // Condition 'day == 0' is always 'false'
        }
    }
}

Mutability

public class HelloWorld {
    public void test(List<String> input) {
        List<String> list = Collections.unmodifiableList(input);
        list.add("Java"); // Immutable object is modified
    }
}

Common dataflow

public class HelloWorld {
    public void test(int x, int y) {
        if (x == y) {
            // Division by zero
            System.out.println(1 / (x - y));
        }
    }
}

public class HelloWorld {
    public void test(List<String> list) {
        if (!list.isEmpty()) return;
        // Collection 'list' is always empty
        for (String s : list) {
            System.out.println(s);
        }
    }
}

public class HelloWorld {
    public long test(int hour, int minutes, int seconds) {
        // hour * 3600: integer multiplication implicitly cast to long
        return hour * 3600 + minutes * 60 + seconds;
    }
}

public class HelloWorld {
    public long test(int hour, int minutes, int seconds) {
        assert hour >= 0 && hour < 24;
        assert minutes >= 0 && minutes < 60;
        assert seconds >= 0 && seconds < 60;
        return hour * 3600 + minutes * 60 + seconds;
    }
}

在return语句中，选中hour * 3600 + minutes * 60 + seconds部分，按下两次Ctrl + Shift + P，可以查看这个表达式的Range。

Dataflow in quick-fixes

优化之前：

import java.util.List;

public class HelloWorld {
    public void test(List<String> list, int maxSize) {
        // Can be replaced with 'List.subList().clear()'
        for (int i = list.size() - 1; i > maxSize; i--) {
            list.remove(i);
        }
    }
}

优化之后：

import java.util.List;

public class HelloWorld {
    public void test(List<String> list, int maxSize) {
        if (list.size() > maxSize + 1) {
            list.subList(maxSize + 1, list.size()).clear();
        }
    }
}

import java.util.List;

public class HelloWorld {
    public void test(List<String> list, int maxSize) {
        if (maxSize >= list.size()) return;
        // Can be replaced with 'List.subList().clear()'
        for (int i = list.size() - 1; i > maxSize; i--) {
            list.remove(i);
        }
    }
}

import java.util.List;

public class HelloWorld {
    public void test(List<String> list, int maxSize) {
        if (maxSize >= list.size()) return;
        if (list.size() > maxSize + 1) {
            list.subList(maxSize + 1, list.size()).clear();
        }
    }
}

考虑是否能做到

public class HelloWorld {
    public int val;

    public int getVal() {
        return val;
    }

    public void test() {
        this.val = getVal(); // 取自己的值，然后赋值给自己。
    }
}

Conclusion

Data flow analysis performs abstract interpretation of your code
It tracks constant values, ranges, oddity, types, nullability, mutability, variable equality and so on
It knows about behavior of many library methods
See how data flow analysis works using advanced expression type feature (Ctrl + Shift + P several times)
Data flow analysis is everywhere: inspections, quick-fixes, completions, debugger, advanced expression type, refactorings, “dataflow to here”.