编译器优化

int a = 1;  
boolean b = false;  
char c = 2; 

int d = a + c;  
int d = b + c;  
char d = a + c; 

public static void main(String[] args) {  
    Map<String, String> map = new HashMap<>();  
    map.put("hello", "你好");  
    map.put("how are you?", "吃了没？");  
    System.out.println(map.get("hello"));  
    System.out.println(map.get("how are you?"));  
}  

public class GenericTypes {  
    public static void method(List<String> list) {  
        System.out.println("invoke method(List<String> list)");  
    }  

    public static void method(List<Integer> list) {  
        System.out.println("invoke method(List<Integer> list)");  
    }  
}  

public class GenericTypes {  
    public static String method(List<String> list) {  
        System.out.println("invoke method(List<String> list)");  
        return "";  
    }

    public static int method(List<Integer> list) {  
        System.out.println("invoke method(List<Integer> list)");  
        return 1;  
    }

    public static void main(String[] args) {  
        method(new ArrayList<String>());  
        method(new ArrayList<Integer>());  
    }  

}  

public static void main(String[] args) {  
    List<Integer> list = Arrays.asList(1, 2, 3, 4);  
    // 如果在 JDK 1.8 中，还有另外一颗语法糖  
    // 能让上面这句代码进一步简写成 List<Integer> list = [1, 2, 3, 4];  
    int sum = 0;  
    for (int i : list) {  
        sum += i;  
    }  
    System.out.println(sum);  
}  

public static void main(String[] args) {
        List list = Arrays.asList(new Integer[]{
            Integer.valueOf(1),
            Integer.valueOf(2),
            Integer.valueOf(3),
            Integer.valueOf(4)
        });

        int sum = 0;
        for(Iterator localIterator = list.iterator(); localIterator.hasNext();){
        int i = ((Integer) localIterator.next()).intValue();
        sum += i;
        }
        System.out.println(sum);
}

public static void main(String[] args) {  
    Integer a = 1;  
    Integer b = 2;  
    Integer c = 3;  
    Integer d = 3;  
    Integer e = 321;  
    Integer f = 321;  
    Long g = 3L;  
    System.out.println(c == d); // true  
    System.out.println(e == f);  // false 
    System.out.println(c == (a + b));  // true 
    System.out.println(c.equals(a + b));  // true 
    System.out.println(g == (a + b));  // true 
    System.out.println(g.equals(a + b));  // false 
}

public static void main(String[] args) {  
    if (true) {  
        System.out.println("block 1");  
    } else {  
        System.out.println("block 2");  
    }  
} 

public static void main(String[] args) {  
    System.out.println("block 1");  
} 

public static void main(String[] args) {  
    // 编译器将会提示 "Unreachable code"  
    while (false) {  
        System.out.println("");  
    }  
} 

import javax.annotation.processing.AbstractProcessor;
import javax.annotation.processing.ProcessingEnvironment;
import javax.annotation.processing.RoundEnvironment;
import javax.annotation.processing.SupportedAnnotationTypes;
import javax.lang.model.element.Element;
import javax.lang.model.element.TypeElement;
import java.util.Set;

// 可以用"*"表示支持所有Annotations
@SupportedAnnotationTypes("*")
//@SupportedSourceVersion(SourceVersion.RELEASE_7)
public class NameCheckProcessor extends AbstractProcessor {

    private NameChecker nameChecker;

    /**
     * 初始化名称检查插件
     */
    @Override
    public void init(ProcessingEnvironment processingEnv) {
        super.init(processingEnv);
        nameChecker = new NameChecker(processingEnv);
    }

    /**
     * 对输入的语法树的各个节点进行进行名称检查
     */
    @Override
    public boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) {
        if (!roundEnv.processingOver()) {
            for (Element element : roundEnv.getRootElements())
                nameChecker.checkNames(element);
        }
        return false;
    }

}

import javax.annotation.processing.Messager;
import javax.annotation.processing.ProcessingEnvironment;
import javax.lang.model.element.*;
import javax.lang.model.util.ElementScanner8;

import java.util.EnumSet;

import static com.sun.tools.javac.parser.Tokens.TokenKind.*;
import static javax.lang.model.element.ElementKind.*;
import static javax.lang.model.element.ElementKind.INTERFACE;
import static javax.tools.Diagnostic.Kind.WARNING;

/**
 * 程序名称规范的编译器插件：<br>
 * 如果程序命名不合规范，将会输出一个编译器的WARNING信息
 */
class NameChecker {
    private final Messager messager;

    private NameCheckScanner nameCheckScanner = new NameCheckScanner();

    NameChecker(ProcessingEnvironment processsingEnv) {
        this.messager = processsingEnv.getMessager();
    }

    /**
     * 对Java程序命名进行检查，根据《Java语言规范》第三版第6.8节的要求，Java程序命名应当符合下列格式：
     *
     * <ul>
     * <li>类或接口：符合驼式命名法，首字母大写。
     * <li>方法：符合驼式命名法，首字母小写。
     * <li>字段：
     * <ul>
     * <li>类、实例变量: 符合驼式命名法，首字母小写。
     * <li>常量: 要求全部大写。
     * </ul>
     * </ul>
     */
    void checkNames(Element element) {
        nameCheckScanner.scan(element);
    }

    /**
     * 名称检查器实现类，继承了JDK 8中新提供的ElementScanner8<br>
     * 将会以Visitor模式访问抽象语法树中的元素
     */
    private class NameCheckScanner extends ElementScanner8<Void, Void> {

        /**
         * 此方法用于检查Java类
         */
        @Override
        public Void visitType(TypeElement e, Void p) {
            scan(e.getTypeParameters(), p);
            checkCamelCase(e, true);
            super.visitType(e, p);
            return null;
        }

        /**
         * 检查方法命名是否合法
         */
        @Override
        public Void visitExecutable(ExecutableElement e, Void p) {
            if (e.getKind() == METHOD) {
                Name name = e.getSimpleName();
                if (name.contentEquals(e.getEnclosingElement().getSimpleName()))
                    messager.printMessage(WARNING, "一个普通方法 “" + name + "”不应当与类名重复，避免与构造函数产生混淆", e);
                checkCamelCase(e, false);
            }
            super.visitExecutable(e, p);
            return null;
        }

        /**
         * 检查变量命名是否合法
         */
        @Override
        public Void visitVariable(VariableElement e, Void p) {
            // 如果这个Variable是枚举或常量，则按大写命名检查，否则按照驼式命名法规则检查
            if (e.getKind() == ENUM_CONSTANT || e.getConstantValue() != null || heuristicallyConstant(e))
                checkAllCaps(e);
            else
                checkCamelCase(e, false);
            return null;
        }

        /**
         * 判断一个变量是否是常量
         */
        private boolean heuristicallyConstant(VariableElement e) {
            if (e.getEnclosingElement().getKind() == INTERFACE)
                return true;
            else
                return e.getKind() == FIELD && e.getModifiers().containsAll(EnumSet.of(PUBLIC, STATIC, FINAL));
        }

        /**
         * 检查传入的Element是否符合驼式命名法，如果不符合，则输出警告信息
         */
        private void checkCamelCase(Element e, boolean initialCaps) {
            String name = e.getSimpleName().toString();
            boolean previousUpper = false;
            boolean conventional = true;
            int firstCodePoint = name.codePointAt(0);

            if (Character.isUpperCase(firstCodePoint)) {
                previousUpper = true;
                if (!initialCaps) {
                    messager.printMessage(WARNING, "名称“" + name + "”应当以小写字母开头", e);
                    return;
                }
            } else if (Character.isLowerCase(firstCodePoint)) {
                if (initialCaps) {
                    messager.printMessage(WARNING, "名称“" + name + "”应当以大写字母开头", e);
                    return;
                }
            } else
                conventional = false;

            if (conventional) {
                int cp = firstCodePoint;
                for (int i = Character.charCount(cp); i < name.length(); i += Character.charCount(cp)) {
                    cp = name.codePointAt(i);
                    if (Character.isUpperCase(cp)) {
                        if (previousUpper) {
                            conventional = false;
                            break;
                        }
                        previousUpper = true;
                    } else
                        previousUpper = false;
                }
            }

            if (!conventional)
                messager.printMessage(WARNING, "名称“" + name + "”应当符合驼式命名法（Camel Case Names）", e);
        }

        /**
         * 大写命名检查，要求第一个字母必须是大写的英文字母，其余部分可以是下划线或大写字母
         */
        private void checkAllCaps(Element e) {
            String name = e.getSimpleName().toString();

            boolean conventional = true;
            int firstCodePoint = name.codePointAt(0);

            if (!Character.isUpperCase(firstCodePoint))
                conventional = false;
            else {
                boolean previousUnderscore = false;
                int cp = firstCodePoint;
                for (int i = Character.charCount(cp); i < name.length(); i += Character.charCount(cp)) {
                    cp = name.codePointAt(i);
                    if (cp == (int) '_') {
                        if (previousUnderscore) {
                            conventional = false;
                            break;
                        }
                        previousUnderscore = true;
                    } else {
                        previousUnderscore = false;
                        if (!Character.isUpperCase(cp) && !Character.isDigit(cp)) {
                            conventional = false;
                            break;
                        }
                    }
                }
            }

            if (!conventional)
                messager.printMessage(WARNING, "常量“" + name + "”应当全部以大写字母或下划线命名，并且以字母开头", e);
        }
    }
}

public class BADLY_NAMED_CODE {

    enum colors {
        red, blue, green;
    }

    static final int _FORTY_TWO = 42;

    public static int NOT_A_CONSTANT = _FORTY_TWO;

    protected void BADLY_NAMED_CODE() {
        return;
    }

    public void NOTcamelCASEmethodNAME() {
        return;
    }
}

klose@ubuntu16-188 ~/tmp/src $ javac com/sun/tools/javac/main/NameChecker.java

klose@ubuntu16-188 ~/tmp/src $ javac com/sun/tools/javac/main/NameCheckProcessor.java

klose@ubuntu16-188 ~/tmp/src $ javac -processor  com.sun.tools.javac.main.NameCheckProcessor BADLY_NAMED_CODE.java

warning: No SupportedSourceVersion annotation found on com.sun.tools.javac.main.NameCheckProcessor, returning RELEASE_6.
warning: Supported source version 'RELEASE_6' from annotation processor 'com.sun.tools.javac.main.NameCheckProcessor' less than -source '1.8'
BADLY_NAMED_CODE.java:1: warning: 名称“BADLY_NAMED_CODE”应当符合驼式命名法（Camel Case Names）
public class BADLY_NAMED_CODE {
       ^
BADLY_NAMED_CODE.java:3: warning: 名称“colors”应当以大写字母开头
        enum colors {
        ^
BADLY_NAMED_CODE.java:4: warning: 常量“red”应当全部以大写字母或下划线命名，并且以字母开头
                red, blue, green;
                ^
BADLY_NAMED_CODE.java:4: warning: 常量“blue”应当全部以大写字母或下划线命名，并且以字母开头
                red, blue, green;
                     ^
BADLY_NAMED_CODE.java:4: warning: 常量“green”应当全部以大写字母或下划线命名，并且以字母开头
                red, blue, green;
                           ^
BADLY_NAMED_CODE.java:7: warning: 常量“_FORTY_TWO”应当全部以大写字母或下划线命名，并且以字母开头
        static final int _FORTY_TWO = 42;
                         ^
BADLY_NAMED_CODE.java:9: warning: 名称“NOT_A_CONSTANT”应当以小写字母开头
        public static int NOT_A_CONSTANT = _FORTY_TWO;
                          ^
BADLY_NAMED_CODE.java:11: warning: 一个普通方法 “BADLY_NAMED_CODE”不应当与类名重复，避免与构造函数产生混淆
        protected void BADLY_NAMED_CODE() {
                       ^
BADLY_NAMED_CODE.java:11: warning: 名称“BADLY_NAMED_CODE”应当以小写字母开头
        protected void BADLY_NAMED_CODE() {
                       ^
BADLY_NAMED_CODE.java:15: warning: 名称“NOTcamelCASEmethodNAME”应当以小写字母开头
        public void NOTcamelCASEmethodNAME() {
                    ^
12 warnings