[Guava Study Notes] Basic Utilities: Null, precondition, Object method, sorting, exception, guava Study Notes

[Guava Study Notes] Basic Utilities: Null, precondition, Object method, sorting, exception, guava Study Notes

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Null

Guava uses Optional to indicate T-type references that may be null.

Create:Optional. of (T) does not accept nullOptional. fromNullable (T) accepts nullOptional. absent ()

Query:IsPresent ()Get ()Or (T) if it is null, It is TOrNull ()

Others:Objects. firstNonNull (T, T) is used to specify the default value for null. If both are null, NullPointerException is thrown.

　　　　EmptyToNull (String ),NullToEmpty (String)

Prerequisites

Usage:

checkArgument(i >= 0, "Argument was %s but expected nonnegative", i);checkArgument(i < j, "Expected i < j, but %s > %s", i, j);

CheckArgument (boolean)CheckNotNull (T)CheckState (boolean)

CheckElementIndex (int index, int size)CheckPositionIndexes (int start, int end, int size)

Inline use:This. field = checkNotNull (field)

Object Method

Objects. equals (a, B) equals that can be nullObjects. hash (Object ...)

ToString ():

// Returns "ClassName{x=1}"Objects.toStringHelper(this).add("x", 1).toString();// Returns "MyObject{x=1}"Objects.toStringHelper("MyObject").add("x", 1).toString();

Compare ():

public int compareTo(Foo that) {    return ComparisonChain.start()            .compare(this.aString, that.aString)            .compare(this.anInt, that.anInt)            .compare(this.anEnum, that.anEnum, Ordering.natural().nullsLast())            .result();}

Sort

Ordering: Comparator implementation

Structure: natural order of natural ()UsingToString () Lexicographic OrderFrom (Comparator)

Chain call:Reverse ()NullsFirst ()NullsLast ()Lexicographical ()OnResultOf (Function)

Ordering<Foo> ordering = Ordering.natural().nullsFirst().onResultOf(new Function<Foo, String>() {  public String apply(Foo foo) {    return foo.sortedBy;  }});

Set and element:GreatestOf (Iterable iterable, int k)Min (Iterable)Min (E, E ...)

IsOrdered (Iterable)SortedCopy (Iterable)