Show Java Settings as Default

Java -XshowSettings can show possible category arguments in JVM env.

Shows settings and continues. Possible category arguments for this option include the following:

Shows all categories of settings. This is the default value.

Shows settings related to locale.

Shows settings related to system properties.

Shows the settings of the JVM.

java -XshowSettings:all -version

when executed above command, the usage information for running the Java launcher will also be displayed. It can make maintainer a bit less convenient to see those details.

VM settings:
    Max. Heap Size (Estimated): 3.56G
    Ergonomics Machine Class: server
    Using VM: Java HotSpot(TM) 64-Bit Server VM

Property settings:
    awt.toolkit = sun.lwawt.macosx.LWCToolkit
    file.encoding = UTF-8
    file.encoding.pkg = sun.io
    file.separator = /
    ftp.nonProxyHosts = local|*.local|169.254/16|*.169.254/16
    gopherProxySet = false
    http.nonProxyHosts = local|*.local|169.254/16|*.169.254/16
    java.awt.graphicsenv = sun.awt.CGraphicsEnvironment
    java.awt.printerjob = sun.lwawt.macosx.CPrinterJob
    java.class.path = .
    java.class.version = 52.0
    java.endorsed.dirs = /Library/Java/JavaVirtualMachines/jdk1.8.0_111.jdk/Contents/Home/jre/lib/endorsed
    java.ext.dirs = /Users/chliu/Library/Java/Extensions
    java.home = /Library/Java/JavaVirtualMachines/jdk1.8.0_111.jdk/Contents/Home/jre
    java.io.tmpdir = /var/folders/zd/5kbv_wbd6cqbpfrf8w95sfnr0000gn/T/
    java.library.path = /Users/chliu/Library/Java/Extensions
    java.runtime.name = Java(TM) SE Runtime Environment
    java.runtime.version = 1.8.0_111-b14
    java.specification.name = Java Platform API Specification
    java.specification.vendor = Oracle Corporation
    java.specification.version = 1.8
    java.vendor = Oracle Corporation
    java.vendor.url = http://java.oracle.com/
    java.vendor.url.bug = http://bugreport.sun.com/bugreport/
    java.version = 1.8.0_111
    java.vm.info = mixed mode
    java.vm.name = Java HotSpot(TM) 64-Bit Server VM
    java.vm.specification.name = Java Virtual Machine Specification
    java.vm.specification.vendor = Oracle Corporation
    java.vm.specification.version = 1.8
    java.vm.vendor = Oracle Corporation
    java.vm.version = 25.111-b14
    line.separator = \n
    os.arch = x86_64
    os.name = Mac OS X
    os.version = 10.14.2
    path.separator = :
    socksNonProxyHosts = local|*.local|169.254/16|*.169.254/16
    sun.arch.data.model = 64
    sun.boot.class.path = /Library/Java/JavaVirtualMachines/jdk1.8.0_111.jdk/Contents/Home/jre/lib/resources.jar
    sun.boot.library.path = /Library/Java/JavaVirtualMachines/jdk1.8.0_111.jdk/Contents/Home/jre/lib
    sun.cpu.endian = little
    sun.cpu.isalist =
    sun.io.unicode.encoding = UnicodeBig
    sun.java.launcher = SUN_STANDARD
    sun.jnu.encoding = UTF-8
    sun.management.compiler = HotSpot 64-Bit Tiered Compilers
    sun.os.patch.level = unknown
    user.country = US
    user.country.format = TW
    user.dir = /Users/chliu/temp/aa/WEB-INF/lib
    user.home = /Users/chliu
    user.language = en
    user.name = chliu
    user.timezone =

Locale settings:
    default locale = English
    default display locale = English (United States)
    default format locale = English (Taiwan)
    available locales = , ar, ar_AE, ar_BH, ar_DZ, ar_EG, ar_IQ, ar_JO,
        ar_KW, ar_LB, ar_LY, ar_MA, ar_OM, ar_QA, ar_SA, ar_SD,
        ar_SY, ar_TN, ar_YE, be, be_BY, bg, bg_BG, ca,
        ca_ES, cs, cs_CZ, da, da_DK, de, de_AT, de_CH,
        de_DE, de_GR, de_LU, el, el_CY, el_GR, en, en_AU,
        en_CA, en_GB, en_IE, en_IN, en_MT, en_NZ, en_PH, en_SG,
        en_US, en_ZA, es, es_AR, es_BO, es_CL, es_CO, es_CR,
        es_CU, es_DO, es_EC, es_ES, es_GT, es_HN, es_MX, es_NI,
        es_PA, es_PE, es_PR, es_PY, es_SV, es_US, es_UY, es_VE,
        et, et_EE, fi, fi_FI, fr, fr_BE, fr_CA, fr_CH,
        fr_FR, fr_LU, ga, ga_IE, hi, hi_IN, hr, hr_HR,
        hu, hu_HU, in, in_ID, is, is_IS, it, it_CH,
        it_IT, iw, iw_IL, ja, ja_JP, ja_JP_JP_#u-ca-japanese, ko, ko_KR,
        lt, lt_LT, lv, lv_LV, mk, mk_MK, ms, ms_MY,
        mt, mt_MT, nl, nl_BE, nl_NL, no, no_NO, no_NO_NY,
        pl, pl_PL, pt, pt_BR, pt_PT, ro, ro_RO, ru,
        ru_RU, sk, sk_SK, sl, sl_SI, sq, sq_AL, sr,
        sr_BA, sr_BA_#Latn, sr_CS, sr_ME, sr_ME_#Latn, sr_RS, sr_RS_#Latn, sr__#Latn,
        sv, sv_SE, th, th_TH, th_TH_TH_#u-nu-thai, tr, tr_TR, uk,
        uk_UA, vi, vi_VN, zh, zh_CN, zh_HK, zh_SG, zh_TW

java version "1.8.0_111"
Java(TM) SE Runtime Environment (build 1.8.0_111-b14)
Java HotSpot(TM) 64-Bit Server VM (build 25.111-b14, mixed mode)


[Java8] Method References

Method references are the special form of Lambda expression. Sometimes, however, a lambda expression does nothing but call an existing method. In those cases, this way is often clearer to refer to existing method by name. They are more compact and easy-to-ready to express something.

Basically, this is just shorthand syntax for a Lambda Expression but this version is more concise & readable.

Types of Method reference

There are four types of method reference.

Reference to a static method

   public void testReferenceStaticMethod()
      Function<String, String> upperFunc = StringUtils::toUpperCase;
      Assert.assertEquals("HI JAVA", upperFunc.apply("hi Java"));

Reference to an instance method of a particular object

   public void testInstanceMethod()
      String text = "hi java";
      Supplier<Integer> getLength = text::length;
      Assert.assertEquals(Integer.valueOf(7), getLength.get());

Reference to an instance method of a particular type

   public void testInstanceMethodByType()
      Function<String, String> upperFunc = String::toLowerCase;
      Assert.assertEquals("hi java", upperFunc.apply("hi Java"));

Reference to a constructor

    public void testCallConstructor()
      Function<Integer, Integer> f = Integer::new;
      Assert.assertEquals(new Integer(99), f.apply(99));


In conclusion, if possible, we should try to use method references can make our code cleaner, more readable & promote code reusability, but they have some restrictions on some cases.


[Java8] the compiled bytecode of lambda

What's byecode of lambda be generated in Java is interesting for me. From below simple code, we can execute javap to get the compiled asm code which will be performed by JVM.

   private Function<String, Integer> f = s -> Integer.parseInt(s);
   public String test()
      Function<String, String> c = (a) -> a.toUpperCase();
      Supplier<Date> s = () -> new Date();
      return c.apply("java") + s.get() + f.apply("99");

javap -c -p LambdaBytecode.class

public class org.javasafari.lambda.LambdaBytecode {
  private java.util.function.Function<java.lang.String, java.lang.Integer> f;

  public org.javasafari.lambda.LambdaBytecode();
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: aload_0
       5: invokedynamic #2,  0              // InvokeDynamic #0:apply:()Ljava/util/function/Function;
      10: putfield      #3                  // Field f:Ljava/util/function/Function;
      13: return

  public java.lang.String test();
       0: invokedynamic #4,  0              // InvokeDynamic #1:apply:()Ljava/util/function/Function;
       5: astore_1
       6: invokedynamic #5,  0              // InvokeDynamic #2:get:()Ljava/util/function/Supplier;
      11: astore_2
      12: new           #6                  // class java/lang/StringBuilder
      15: dup
      16: invokespecial #7                  // Method java/lang/StringBuilder."<init>":()V
      19: aload_1
      20: ldc           #8                  // String java
      22: invokeinterface #9,  2            // InterfaceMethod java/util/function/Function.apply:(Ljava/lang/Object;)Ljava/lang/Object;
      27: checkcast     #10                 // class java/lang/String
      30: invokevirtual #11                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      33: aload_2
      34: invokeinterface #12,  1           // InterfaceMethod java/util/function/Supplier.get:()Ljava/lang/Object;
      39: invokevirtual #13                 // Method java/lang/StringBuilder.append:(Ljava/lang/Object;)Ljava/lang/StringBuilder;
      42: aload_0
      43: getfield      #3                  // Field f:Ljava/util/function/Function;
      46: ldc           #14                 // String 99
      48: invokeinterface #9,  2            // InterfaceMethod java/util/function/Function.apply:(Ljava/lang/Object;)Ljava/lang/Object;
      53: invokevirtual #13                 // Method java/lang/StringBuilder.append:(Ljava/lang/Object;)Ljava/lang/StringBuilder;
      56: invokevirtual #15                 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
      59: areturn

  private static java.util.Date lambda$test$2();
       0: new           #16                 // class java/util/Date
       3: dup
       4: invokespecial #17                 // Method java/util/Date."<init>":()V
       7: areturn

  private static java.lang.String lambda$test$1(java.lang.String);
       0: aload_0
       1: invokevirtual #18                 // Method java/lang/String.toUpperCase:()Ljava/lang/String;
       4: areturn

  private static java.lang.Integer lambda$new$0(java.lang.String);
       0: aload_0
       1: invokestatic  #19                 // Method java/lang/Integer.parseInt:(Ljava/lang/String;)I
       4: invokestatic  #20                 // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
       7: areturn

Lambdas and invokedynamic

The translation of a lambda expression to be bytecode will be performed in below steps.

  • Generate an invokedynamic call site (lambda factory), which when invoked returns an instance of the Functional Interface defined.
  • Create a static method which included the lambda expression will be invoked through the invokedynamic instruction.

If you want to know how invokedynamic works, "Using Invoke Dynamic to Teach the JVM a New Language" is good for us.


[Java8] Lambda Expression

From Wikipedia, the free encyclopedia

In computer programming, an anonymous function (function literal, lambda abstraction, or lambda expression) is a function definition that is not bound to an identifier. Anonymous functions are often arguments being passed to higher-order functions, or used for constructing the result of a higher-order function that needs to return a function.[1] If the function is only used once, or a limited number of times, an anonymous function may be syntactically lighter than using a named function. Anonymous functions are ubiquitous in functional programming languages and other languages with first-class functions, where they fulfill the same role for the function type as literals do for other data types.

What's a lambda expression in Java world?
A lambda expression is an unnamed block of code with a list of formal parameters and a body, or named an anonymous function.
It can be easily imagined with 3 parts: arguments list, arrow token and body. Let's see more examples.

() -> {}                // No parameters; result is void
() -> 42                // No parameters, expression body
() -> null              // No parameters, expression body
() -> { return 42; }    // No parameters, block body with return
() -> { System.gc(); }  // No parameters, void block body
() -> {                 // Complex block body with returns
  if (true) return 12;
  else {
    int result = 15;
    for (int i = 1; i < 10; i++)
      result *= i;
    return result;
} }
(int x) -> x+1              // Single declared-type parameter
(int x) -> { return x+1; }  // Single declared-type parameter
(x) -> x+1
x -> x+1
(String s) -> s.length()
(Thread t) -> { t.start(); }  // Single declared-type parameter
s -> s.length()               // Single inferred-type parameter
t -> { t.start(); }           // Single inferred-type parameter

(int x, int y) -> x+y  // Multiple declared-type parameters
(x, y) -> x+y          // Multiple inferred-type parameters
(x, int y) -> x+y    // Illegal: can't mix inferred and declared types
(x, final y) -> x+y  // Illegal: no modifiers with inferred types

Basically, we should use inferred-type parameters and simple statement as body while writing code.


[Java8] Map with Mew Default Methods

Java 8 introduced the new default methods to Map interface which stores lazied-computed value and doesn't break map contract. We can be based on below to write clean code.


Returns the value to which the specified key is mapped, or defaultValue if this map contains no mapping for the key.

   public void testGetOfDefault()
      Map<String, Integer> studentGrades = new HashMap<>();
      Assert.assertThat(studentGrades.getOrDefault("Chris", 0), equalTo(new Integer(0)));


Performs the give action for each entry in this map util all entries have been processed or the action throws an exception.

   public void testForEach()
      Map<String, Integer> collector = new HashMap()
      { { put("A", 99); put("B", 80); put("C", 70); } };
      collector.forEach((key, value) -> System.out.println(String.format("%s => %s", key, value)));


Replaces each entry's value with the result of invoking the given function on that entry util all entries have been processed or the function throws an exception.

   public void testReplaceAll()
      Map<String, Integer> collector = new HashMap()
      { { put("A", 99); put("B", 82); put("C", 71); } };
      collector.replaceAll((key, value) -> value < 90 ? value + 10 : value);
      assertThat(collector, hasEntry("A", 99));
      assertThat(collector, hasEntry("B", 92));
      assertThat(collector, hasEntry("C", 81));


If the specified key is not already associated with a value is mapped to null associates if with the given value and return null, else returns the current value.

   public void testPutIfAbsent()
      Map<String, Integer> counters = new HashMap()
      {{ put("c0", 0); }};

      counters.putIfAbsent("c0", 99);
      counters.putIfAbsent("c1", 99);
      assertEquals(Integer.valueOf(0), counters.get("c0"));
      assertEquals(Integer.valueOf(99), counters.get("c1"));


If the specified key is not already associated with a value, attempts too compute its value using the given function and enters it into this map unless null.

   public void testComputeIfAbsent()
      Map<String, List<Integer>> counters = new HashMap<>();
      counters.computeIfAbsent("key", k -> new ArrayList<>()).addAll(Arrays.asList(1, 3, 5));
      Assert.assertThat(counters.get("key"), hasItems(1, 3, 5));


If the value for the specified key is present and not-null, attempts to compute a new mapping given the key and its current mapped value.

   public void testComputeIfPresent()
      Map<String, Integer> map = new HashMap()
         put("key", 2);
      map.computeIfPresent("key", (k, v) -> v / 2);
      assertThat(map.get("key"), equalTo(1));


If the value for the specified key is present and non-null, attempts to compute a new mapping give the key and its current mapped value.

   public void testMergeValue()
      Map<String, String> holder = new HashMap();
      holder.put("chris", "^^~");
      holder.merge("chris", "Java", (v1, v2) -> v1.concat(v2));
      Assert.assertThat(holder.get("chris"), is("^^~Java"));


[Java8] Optional

Java 8 SE introduces a new class called Optional that is inspired from Haskell and Scala. It's a class that encapsulates an option value. We can view Optional as a single value that either contains a value or not.

From Java definition

A container object which may or may not contain a non-null value. If a value is present, isPresent() will return true and get() will return the value.

Here are some test cases.

Creates the non-null and null value optional.

   public void testCreate_NonNull_null_Optionals()
      Optional<String> country = Optional.of("Taiwan");
      Assertions.assertEquals("Taiwan", country.get());

      Assertions.assertEquals(false, Optional.ofNullable(null).isPresent());
      Assertions.assertEquals("defaultValue", Optional.ofNullable(null).orElse("defaultValue"));

Return value if present, otherwise return other.

   public void testGetElseOption()
      Optional<Date> current = Optional.ofNullable(null);
      Assertions.assertNotNull(current.orElse(new Date()));

Return the value if present, otherwise invoke other and return the result of that invocation.

   public void testOrElseGet()
      Date dateNow = null;

Return the contained value, if present, otherwise throw an exception to be created by the provided supplier.

   public void testOrElseThrow()
      Date dateNow = null;
         () -> Optional.ofNullable(dateNow).orElseThrow(IllegalArgumentException::new));

If a value is present, and the value matches the given predicate, return a describing the value, otherwise return an empty.

   public void testFilter()
      Optional<String> taipeiCity = Optional.of("Taipei").filter((v) -> v.contains("No.1 City"));
      Assertions.assertEquals(Optional.empty(), taipeiCity);

      Optional<String> county = Optional.of("Taiwan").filter(v -> v.contains("Taiwan"));
      Assertions.assertEquals("Taiwan", county.get());

If a value is present, apply the provided {@code Optional}-bearing mapping function to it, return that result, otherwise return an empty {@code Optional}

   public void testFlatMap()
      Optional<Integer> numOp = Optional.of("111").flatMap(v -> Optional.of(Integer.valueOf(v)));
      Assertions.assertEquals(Integer.valueOf(111), numOp.get());

In Java8 Optional is often be misused, optionals are not as easy as they seem. This tutorial will help us to avoid doing stupid code.

26 Reasons Why Using Optional Correctly Is Not Optional


[Java8] BiFunction Functional Interface

From Wikipedia about Arity,

In logic, mathematics, and computer science, the arity /ˈærɪti/ (About this soundlisten) of a function or operation is the number of arguments or operands that the function takes

A binary function takes two arguments.
Example: {f(x,y)=2xy}

From Java definition

Represents a function that accepts two arguments and produces a result. This is the two arity specialization of Function.

here are some test cases to understand deeply biFunction.

   public void testFindEvenNumbers()

      BiFunction<List<Integer>, Predicate<Integer>, List<Integer>> evenFinder = (list, even) -> {
         return list.stream().filter(even).collect(Collectors.toList());

      List<Integer> evenNumList = evenFinder.apply(
         Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8),
         (num) -> num % 2 == 0

      MatcherAssert.assertThat(evenNumList, hasItems(2, 4, 6, 8));